[PowerPC] Improve handling of some BUILD_VECTOR nodes

An analysis of real world code turned up a number of patterns with BUILD_VECTOR
of nodes resulting from operations on extracted vector elements for which we
produce poor code. This addresses those cases. No attempt is made for
completeness as that would entail a large amount of work for something that
there is no evidence of in real code.

Differential revision: https://reviews.llvm.org/D72660