[GlobalIsel][X86] selectDivRem - fix typo in 64-bit AH handling code

This function was lifted from fast-isel, and still referred to the Instruction::SRem/URrem opcodes, instead of the G_SREM/G_UREM opcodes.

But it turns out these aren't necessary at all as only the G_SREM/G_UREM codepaths will use the AH register for DivRemResultReg anyhow.