[NFC] [PowerPC] Narrow fast-math flags in tests

A lot of tests under PowerPC are using fast flag, while fast is just
alias of 7 fast-math flags. This change makes test points clearer.

mc-instrlat.ll and sms-iterator.ll keeps unchanged since they are not
testing fast-math behavior. (one for machine combiner crash, one for
machine pipeliner bug)

Reviewed By: steven.zhang, spatel

Differential Revision: https://reviews.llvm.org/D78989