[PowerPC] Eliminate integer compare instructions - vol. 1

This patch is the first in a series of patches to provide code gen for
doing compares in GPRs when the compare result is required in a GPR.

It adds the infrastructure to select GPR sequences for i1->i32 and i1->i64
extensions. This first patch handles equality comparison on i32 operands with
the result sign or zero extended.

Differential Revision: https://reviews.llvm.org/D31847

llvm-svn: 302810