[X86][AVX] insert_subvector(bitcast(v), bitcast(s), c1) -> bitcast(insert_subvector(v,s,c2))

Similar to what we already do in DAGCombiner, but this version also handles bitcasts from types with different scalar sizes, which x86 is better at handling.

Differential Revision: https://reviews.llvm.org/D57514

llvm-svn: 352773