[X86] Remove custom lowering vXi1 extending loads and truncating stores.

Summary: Now that v2i1/v4i1 are legal without VLX. And v32i1 is legalized by splitting rather than widening. And isVectorLoadExtDesirable returns false for vXi1. It appears this handling is dead because the operations simply don't exist.

Reviewers: RKSimon, zvi, guyblank, delena, spatel

Reviewed By: delena

Subscribers: llvm-commits, rengolin

Differential Revision: https://reviews.llvm.org/D42781

llvm-svn: 323983