First teach instcombine that sign bit checks only demand the
sign bit, this allows simplify demanded bits to hack on
expressions better.
Second, teach instcombine that ashr is useless if only the
sign bit is demanded.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@39880 91177308-0d34-0410-b5e6-96231b3b80d8
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