[PowerPC] Simplify fp-to-int store optimization

On PowerPC VSX targets, fp-to-int will be transformed into xscv with
mfvsr. When the result is to be stored, mfvsr can be replaced by a
direct store.

This change simplifies the optimization by using existing fp-to-int
code, which helps CSE and handling strictfp cases.

Reviewed By: shchenz

Differential Revision: https://reviews.llvm.org/D141473