[X86] Custom split v32i16/v64i8 bitcasts when AVX512F is available, but BWI is not.

The test changes you can see are related to the changes in ReplaceNodeResults. Though shuffle-vs-trunc-512.ll does have a test that exercises the code in LowerBITCAST. Looks like the test output didn't change because DAG combining is able to clean up the resulting type legalization. Adding the custom hook just makes type legalization work less hard.

Differential Revision: https://reviews.llvm.org/D43447

llvm-svn: 325933