[IndVarSimplify] Widen loop compare instructions.

This improves other optimizations such as LSR.  A sext may be added to the
compare's other operand, but this can often be hoisted outside of the loop.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217953 91177308-0d34-0410-b5e6-96231b3b80d8