The backend generally uses 64-bit immediates (e.g. what MachineOperand::getImm() returns), so use that for analyzeCompare() and optimizeCompareInst() as well. This avoids truncation for targets that support immediates larger 32-bit. In particular, we can avoid the bugprone value normalization hack in the AArch64 target.
This is a followup to D108076.