[X86] Remove portions of r275950 that are no longer needed with i1 not being a legal type

Summary:
r275950 added support for turning (trunc (X >> N) to i1) into BT(X, N). But that's no longer necessary now that i1 isn't legal.

This patch removes the support for that, but preserves some of the refactorings done in that commit.

Reviewers: guyblank, RKSimon, spatel, zvi

Reviewed By: RKSimon

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D37673

llvm-svn: 312925