[AArch64] Merge [US]MULL with half adds and subs into [US]ML[AS]L

This patch adds patterns to teach the AArch64 backend to merge [US]MULL
instructions and adds/subs of half the size into [US]ML[AS]L where we don't use
the top half of the result.

Differential Revision: https://reviews.llvm.org/D95218