[ARM] Favour PL/MI over GE/LT when possible
The arm condition codes for GE is N==V (and for LT is N!=V). If the source of
flags cannot set V (overflow), such as a cmp against #0, then we can use the
simpler PL and MI conditions that only check N. As these PL/MI conditions are
simpler than GE/LT, other passes like the peephole optimiser can have a better
time optimising away the redundant CMPs.
The exception is the VSEL instruction, which cannot take the PL code, so there
the transform favours GE.
Differential Revision: https://reviews.llvm.org/D64160
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@365117
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