[AArch64] Don't rely on (zext (trunc x)) pattern to detect zext_inreg MULL...

[AArch64] Don't rely on (zext (trunc x)) pattern to detect zext_inreg MULL patterns - use value tracking directly

As explained on D159533, I'm trying to generalize the "(zext (trunc x)) -> x iff the upper bits are known zero" fold in getNode() and I was seeing assertions in the aarch64 mull matching code as it was assuming these 'zero-extend-inreg' patterns will remain from earlier in LowerMUL.

Instead I've updated selectUmullSmull/skipExtensionForVectorMULL to just use value tracking to detect when the upper bits are known zero, and to insert the truncation nodes later if necessary.

Differential Revision: https://reviews.llvm.org/D159537