[X86] Remove the SlowBTMem feature flag entirely

[X86] Remove the SlowBTMem feature flag entirely

Turns out we have no patterns on the instructions that were using this feature flag for other reasons. These instructions are slow on all modern CPUs so it seems unlikely that we will spend any effort supporting these instructions going forward. So we might as well just kill of the feature flag and just fix up the comments.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@315862 91177308-0d34-0410-b5e6-96231b3b80d8