Fix PR4753.

Fix PR4753.

When undoing a reuse in ReuseInfo::GetRegForReload, check if it was only a
sub-register being used. The MachineOperand::getSubReg() method is only valid
for virtual registers, so we have to recover the sub-register index manually.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@79855 91177308-0d34-0410-b5e6-96231b3b80d8