[X86] Use ISD::SIGN_EXTEND instead of X86ISD::VSEXT for mask to xmm/ymm/zmm conversion

There are a couple tricky things with this patch.

I had to add an override of isVectorLoadExtDesirable to stop DAG combine from combining sign_extend with loads after legalization since we legalize sextload using a load+sign_extend. Overriding this hook actually prevents a lot sextloads from being created in the first place.

I also had to add isel patterns because DAG combine blindly combines sign_extend+truncate to a smaller sign_extend which defeats what legalization was trying to do.

Differential Revision: https://reviews.llvm.org/D42407

llvm-svn: 323301