[PowerPC] [FPEnv] Disable strict FP mutation by default

22a0edd0 introduced a config IsStrictFPEnabled, which controls the
strict floating point mutation (transforming some strict-fp operations
into non-strict in ISel). This patch disables the mutation by default
since we've finished PowerPC strict-fp enablement in backend.

Reviewed By: uweigand

Differential Revision: https://reviews.llvm.org/D87222