This builds on D27028 and D32319's work on constrained math intrinsics.
Quoting from D32319: "The purpose of the constrained intrinsics is to force the optimizer to respect the restrictions that will be necessary to support things like the STDC FENV_ACCESS ON pragma without interfering with optimizations when these restrictions are not needed."
There are more patches coming, but I wanted to start with just a handful here.