[PowerPC] Custom Lowering BUILD_VECTOR for v2i64 for P7 as well

[PowerPC] Custom Lowering BUILD_VECTOR for v2i64 for P7 as well

The lowering for v2i64 is now guarded with hasDirectMove,
however, the current lowering can handle the pattern correctly,
only lowering it when there is efficient patterns and corresponding
instructions.

The original guard was added in D21135, and was for Legal action.
The code has evloved now, this guard is not necessary anymore.

Reviewed By: #powerpc, nemanjai

Differential Revision: https://reviews.llvm.org/D105596