It turns out most of the thumb2 instructions are not allowed to touch SP. The semanti...

It turns out most of the thumb2 instructions are not allowed to touch SP. The semantics of such instructions are unpredictable. We have just been lucky that tests have been passing.

This patch takes pain to ensure all the PEI lowering code does the right thing when lowering frame indices, insert code to manipulate stack pointers, etc. It's also custom lowering dynamic stack alloc into pseudo instructions so we can insert the right instructions at scheduling time.

This fixes PR4659 and PR4682.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@78361 91177308-0d34-0410-b5e6-96231b3b80d8