Revert r212640, "Add trunc (select c, a, b) -> select c (trunc a), (trunc b) combine."

This caused miscompilation on, at least, x86-64. SExt(i1 cond) confused other optimizations.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212708 91177308-0d34-0410-b5e6-96231b3b80d8
llvm/test/CodeGen/X86/shift-parts.ll: FileCheck-ize. (from r212640)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212709 91177308-0d34-0410-b5e6-96231b3b80d8