As of 8dacca943a, we sign extend the atomic loaded
operand for signed subword comparisons. However, the assumption that the other
operand is correctly sign extended doesn't always hold. This patch sign extends
the other operand if it needs to be sign extended.
This is a second fix for https://bugs.llvm.org/show_bug.cgi?id=30451
Differential revision: https://reviews.llvm.org/D94058
Nemanja Ivanovic