IRGen: minor clean up to explosion handling (NFC)
IRGen: minor clean up to explosion handling (NFC)

Indicate that the type is const for the cast.  Use `Range` and a
for-loop rather than the explicit counters.  NFC.

xwu/swift,shajrawi/swift,jopamer/swift,devincoughlin/swift,shahmishal/swift,shajrawi/swift,zisko/swift,frootloops/swift,ahoppen/swift,brentdax/swift,jopamer/swift,xedin/swift,benlangmuir/swift,sschiau/swift,rudkx/swift,shahmishal/swift,lorentey/swift,ahoppen/swift,hooman/swift,parkera/swift,danielmartin/swift,CodaFi/swift,aschwaighofer/swift,gribozavr/swift,gregomni/swift,harlanhaskins/swift,zisko/swift,tkremenek/swift,ahoppen/swift,devincoughlin/swift,frootloops/swift,nathawes/swift,brentdax/swift,jopamer/swift,huonw/swift,tkremenek/swift,apple/swift,jckarter/swift,roambotics/swift,alblue/swift,austinzheng/swift,shajrawi/swift,jckarter/swift,JGiola/swift,xwu/swift,lorentey/swift,xedin/swift,glessard/swift,glessard/swift,natecook1000/swift,harlanhaskins/swift,parkera/swift,hooman/swift,CodaFi/swift,xwu/swift,benlangmuir/swift,apple/swift,sschiau/swift,zisko/swift,JGiola/swift,xedin/swift,benlangmuir/swift,danielmartin/swift,atrick/swift,rudkx/swift,tkremenek/swift,shajrawi/swift,jmgc/swift,parkera/swift,harlanhaskins/swift,practicalswift/swift,zisko/swift,austinzheng/swift,lorentey/swift,xwu/swift,alblue/swift,alblue/swift,austinzheng/swift,CodaFi/swift,jmgc/swift,natecook1000/swift,allevato/swift,jmgc/swift,apple/swift,gregomni/swift,stephentyrone/swift,aschwaighofer/swift,ahoppen/swift,sschiau/swift,devincoughlin/swift,amraboelela/swift,stephentyrone/swift,sschiau/swift,glessard/swift,shahmishal/swift,lorentey/swift,glessard/swift,rudkx/swift,brentdax/swift,huonw/swift,gregomni/swift,hooman/swift,rudkx/swift,jopamer/swift,sschiau/swift,allevato/swift,shahmishal/swift,gribozavr/swift,shajrawi/swift,nathawes/swift,shajrawi/swift,practicalswift/swift,harlanhaskins/swift,practicalswift/swift,stephentyrone/swift,lorentey/swift,benlangmuir/swift,hooman/swift,jckarter/swift,devincoughlin/swift,parkera/swift,frootloops/swift,brentdax/swift,shahmishal/swift,natecook1000/swift,shajrawi/swift,tkremenek/swift,airspeedswift/swift,CodaFi/swift,alblue/swift,devincoughlin/swift,natecook1000/swift,brentdax/swift,allevato/swift,airspeedswift/swift,lorentey/swift,glessard/swift,jmgc/swift,huonw/swift,jckarter/swift,karwa/swift,danielmartin/swift,allevato/swift,jmgc/swift,xedin/swift,amraboelela/swift,harlanhaskins/swift,alblue/swift,aschwaighofer/swift,practicalswift/swift,gribozavr/swift,gribozavr/swift,harlanhaskins/swift,tkremenek/swift,huonw/swift,devincoughlin/swift,jckarter/swift,tjw/swift,karwa/swift,devincoughlin/swift,CodaFi/swift,xedin/swift,xedin/swift,hooman/swift,lorentey/swift,glessard/swift,sschiau/swift,rudkx/swift,tjw/swift,nathawes/swift,xedin/swift,roambotics/swift,allevato/swift,gregomni/swift,shahmishal/swift,practicalswift/swift,danielmartin/swift,practicalswift/swift,tjw/swift,zisko/swift,xedin/swift,nathawes/swift,gribozavr/swift,airspeedswift/swift,ahoppen/swift,jmgc/swift,hooman/swift,alblue/swift,jopamer/swift,amraboelela/swift,austinzheng/swift,huonw/swift,stephentyrone/swift,jopamer/swift,danielmartin/swift,parkera/swift,allevato/swift,sschiau/swift,atrick/swift,amraboelela/swift,gribozavr/swift,aschwaighofer/swift,nathawes/swift,gribozavr/swift,karwa/swift,jmgc/swift,JGiola/swift,stephentyrone/swift,harlanhaskins/swift,stephentyrone/swift,airspeedswift/swift,austinzheng/swift,shahmishal/swift,jckarter/swift,CodaFi/swift,amraboelela/swift,gregomni/swift,karwa/swift,tjw/swift,airspeedswift/swift,apple/swift,sschiau/swift,ahoppen/swift,danielmartin/swift,JGiola/swift,amraboelela/swift,karwa/swift,natecook1000/swift,aschwaighofer/swift,karwa/swift,karwa/swift,danielmartin/swift,amraboelela/swift,jopamer/swift,benlangmuir/swift,lorentey/swift,tjw/swift,jckarter/swift,natecook1000/swift,frootloops/swift,roambotics/swift,nathawes/swift,shajrawi/swift,atrick/swift,tkremenek/swift,CodaFi/swift,apple/swift,frootloops/swift,tjw/swift,roambotics/swift,austinzheng/swift,JGiola/swift,atrick/swift,gregomni/swift,aschwaighofer/swift,atrick/swift,frootloops/swift,karwa/swift,atrick/swift,xwu/swift,rudkx/swift,gribozavr/swift,airspeedswift/swift,parkera/swift,frootloops/swift,apple/swift,tjw/swift,zisko/swift,huonw/swift,practicalswift/swift,hooman/swift,tkremenek/swift,zisko/swift,benlangmuir/swift,stephentyrone/swift,devincoughlin/swift,parkera/swift,parkera/swift,brentdax/swift,austinzheng/swift,shahmishal/swift,JGiola/swift,huonw/swift,nathawes/swift,xwu/swift,allevato/swift,practicalswift/swift,roambotics/swift,natecook1000/swift,airspeedswift/swift,brentdax/swift,roambotics/swift,xwu/swift,alblue/swift,aschwaighofer/swift