[AArch64] Common patterns between UMULL and int_aarch64_neon_umull

We have some duplicate patterns between the AArch64ISD::UMULL (/SMULL)
and the int_aarch64_neon_umull (/smull) intrinsics. They did not
replicate all the patterns though, leaving some gaps on instructions
like umlal2 from codegen. This commons all the patterns by converting
all int_aarch64_neon_umull intrinsics to UMULL nodes and removing the
duplicate for umull/smull intrinsics, so that all instructions go
through the same tablegen pattern.

This improves some of the longer-than-legal mla patterns, helping them
replace ext with umlal2.

Differential Revision: https://reviews.llvm.org/D119887