[x86/SLH] Teach speculative load hardening to correctly harden the

indices used by AVX2 and AVX-512 gather instructions.

The index vector is hardened by broadcasting the predicate state
into a vector register and then or-ing. We don't even have to worry
about EFLAGS here.

I've added a test for all of the gather intrinsics to make sure that we
don't miss one. A particularly interesting creation is the gather
prefetch, which needs to be marked as potentially "loading" to get the
correct behavior. It's a memory access in many ways, and is actually
relevant for SLH. Based on discussion with Craig in review, I've moved
it to be `mayLoad` and `mayStore` rather than generic side effects. This
matches how we model other prefetch instructions.

Many thanks to Craig for the review here.

Differential Revision: https://reviews.llvm.org/D49336

llvm-svn: 337144