[AArch64] Combine add(extract v1i64) into v1i64 add

This helps fix a regression from D148309 where a shift + add was no longer
combined into a ssra. It looks for add's with v1i64 extract operands and
converts them to v1i64 adds. The other operand needs to be something that is
easily converted to a v1i64, in this case it currently just checks for a load.

Some of the code in performAddSubCombine has been cleaned up whilst I was here.

Differential Revision: https://reviews.llvm.org/D148311