Instead of passing around fast-math-flags as a parameter, we can set those using an IRBuilder guard object. This is no-functional-change-intended, but hopefully someone more familiar with this code can try to confirm that.
The motivation is to eventually fix the vectorizers to use and set the correct fast-math-flags for reductions. Examples of that not behaving as expected are:
https://bugs.llvm.org/show_bug.cgi?id=23116 (should be able to reduce with less than 'fast')
https://bugs.llvm.org/show_bug.cgi?id=35538 (possible miscompile for -0.0)
D61802 (should be able to reduce with IR-level FMF)