[PowerPC] Recommit r314244 with refactoring and off by default

[PowerPC] Recommit r314244 with refactoring and off by default

This re-commits everything that was pulled in r314244. The transformation
is off by default (patch to enable it to follow). The code is refactored
to have a single entry-point and provide fine-grained control over patterns
that it selects. This patch also fixes the bugs in the original code.

Everything that failed with the original patch has been re-tested with this
patch (with the transformation turned on). So the patch to turn this on is
soon to follow.

Differential Revision: https://reviews.llvm.org/D38575

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@319434 91177308-0d34-0410-b5e6-96231b3b80d8