[DAGCombine] Fold insert_subvector(bitcast(x),bitcast(y),c1) -> bitcast(insert_subvector(x,y),c2)

Move this combine from x86 into generic DAGCombine, which currently only manages cases where the bitcast is between types of the same scalarsize.

Differential Revision: https://reviews.llvm.org/D59188

llvm-svn: 362324