Improve codegen for the various "with overflow" intrinsics
Improve codegen for the various "with overflow" intrinsics

We're currently possibly introducing an unneeded temporary, make use of
InsertValue which is said to kick us off of FastISel and we generate
loads/stores of first class aggregates, which is bad as well. Let's not
do all these things.

aidancully/rust,GBGamer/rust,GBGamer/rust,graydon/rust,GBGamer/rust,graydon/rust,GBGamer/rust,GBGamer/rust,graydon/rust,aidancully/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust