[ARM][LowOverheadLoops] Revert after read/write

    
Currently we check whether LR is stored/loaded to/from inbetween the
loop decrement and loop end pseudo instructions. There's two problems
here:
- It relies on all load/store instructions being labelled as such in
  tablegen.
- Actually any use of loop decrement is troublesome because the value
  doesn't exist!
    
So we need to check for any read/write of LR that occurs between the
two instructions and revert if we find anything.

Differential Revision: https://reviews.llvm.org/D65792

llvm-svn: 368130

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