[AVX512] Add patterns to match masked extract_subvector with bitcasts between...

[AVX512] Add patterns to match masked extract_subvector with bitcasts between the vselect and the extract_subvector. Remove the late DAG combine.

We used to do a late DAG combine to move the bitcasts out of the way, but I'm starting to think that it's better to canonicalize extract_subvector's type to match the type of its input. I've seen some cases where we've formed two different extract_subvector from the same node where one had a bitcast and the other didn't.

Add some more test cases to ensure we've also got most of the zero masking covered too.

llvm-svn: 311837