[PowerPC] Simplify fp-to-int store optimization

[qiucf]

Summary

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

This patch simplifies the optimization by using existing fp-to-int code, which also helps for:

1. Strict floating point support of this optimization (in another patch)
2. Better codegen from CSE

[shchenz]

llvm/test/CodeGen/PowerPC/store_fptoi.ll

1032

From the case change, I am thinking common the 3 xscvdpsxds sounds like making more sense?