[DAGCombiner][X86][SystemZ][AArch64] Combine some cases of (bitcast...

[DAGCombiner][X86][SystemZ][AArch64] Combine some cases of (bitcast (build_vector constants)) between legalize types and legalize dag.

This patch enables combining integer bitcasts of integer build vectors when the new scalar type is legal. I've avoided floating point because the implementation bitcasts float to int along the way and we would need to check the intermediate types for legality

Differential Revision: https://reviews.llvm.org/D58884

llvm-svn: 355324