[DAG] Move integer setcc %x, %x folding into FoldSetCC

First step towards PR40800 - I intend to move the float case in a separate future patch.

I had to tweak the (overly reduced) thumb2 test and the x86 widening test change is annoying (no longer rematerializable) but we should address this separately.

Differential Revision: https://reviews.llvm.org/D59244

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@356040 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed