[llvm] GlobalISel: adapt MachineSSAUpdater for use in GlobalISel path (PR #78431)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> > Seems out of date, and needs to swap in G_* opcodes for generic registers
> 
> For global-isel lane-mask registers are not generic registers, they have regclass + LLT instead of only regclass. PHI lane masks should be fully instruction selected (all reg operands need to have reg class) when they are introduced. Undef lane-masks should also be inst-selected, so use of IMPLICIT_DEF is fine.

If that's the case, then this patch is unnecessary? Allowing MachineSSAUpdater in GlobalISel means handling generic virtual registers, and emitting the appropriate G_* opcodes instead of the post-selection equivalents.

https://github.com/llvm/llvm-project/pull/78431