[PowerPC] use inst-level fast-math-flags to drive MachineCombiner

Currently, on PowerPC target, it uses function scope UnsafeFPMath
option to drive Machine Combiner pass.

This is not accurate in two ways:
1: the scope is not accurate. Machine Combiner pass only requires
   instruction-level flags instead of the function scope.
2: the float point flag is not accurate. Machine Combiner pass
   only requires float point flags reassoc and nsz.

Reviewed By: steven.zhang

Differential Revision: https://reviews.llvm.org/D78183