[PPC] Check for PPC64 when emitting 64bit specific VSX nodes when pattern matching built vectors

Some of the pattern matching in PPCInstrVSX.td and node lowering involving vectors assumes 64bit mode.  This patch disables some of the unsafe pattern matching and lowering of BUILD_VECTOR in 32bit mode.

Reviewed By: Xiangling_L

Differential Revision: https://reviews.llvm.org/D92789