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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217019 91177308-0d34-0410-b5e6-96231b3b80d8
Fix PR20800: correctly calculate the offset of the subq instruction when generating compact unwind info.

This CL replaces the constant DarwinX86AsmBackend.PushInstrSize with a method
that lets the backend account for different sizes of "push %reg" instruction
sizes.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217020 91177308-0d34-0410-b5e6-96231b3b80d8