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

[Petar Avramovic via llvm-commits]

petar-avramovic 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 Seems out of date, and needs to swap in G_* opcodes for generic registers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.


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