Reapply r216805 "[MachineCombiner][AArch64] Use the correct register class for MADD, SUB, and OR.""

This reapplies r216805 with a fix to a copy-past error, which resulted in an
incorrect register class.

Original commit message:
Select the correct register class for the various instructions that are
generated when combining instructions and constrain the registers to the
appropriate register class.

This fixes rdar://problem/18183707.