Add some comments to test/IRGen/builtin_math.swift
Add some comments to test/IRGen/builtin_math.swift

This test calls sqrt for both Double and Float values, and those calls get
compiled very differently. Add an explanation of the difference.

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