[PowerPC] Support constrained vector fp/int conversion

This patch makes these operations legal, and add necessary codegen
patterns.

There's still some issue similar to D77033 for conversion from v1i128
type. But normal type tests synced in vector-constrained-fp-intrinsic
are passed successfully.

Reviewed By: uweigand

Differential Revision: https://reviews.llvm.org/D83654