[LIR] Remove the dedicated class for popcount recognition and sink the

code into methods on LoopIdiomRecognize.

This simplifies the code somewhat and also makes it much easier to move
the analyses around. Ultimately, the separate class wasn't providing
significant value over methods -- it contained the precondition basic
block and the current loop. The current loop is already available and
the precondition block wasn't needed everywhere and is easy to pass
around.

In several cases I just moved things to be static functions because they
already accepted most of their inputs as arguments.

This doesn't fix the way we manage analyses yet, that will be the next
patch, but it already makes the code over 50 lines shorter.

No functionality changed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@244851 91177308-0d34-0410-b5e6-96231b3b80d8