[llvm] [CodeGen] Add generic INIT_UNDEF pseudo (PR #106744)

[Jack Styles via llvm-commits]

Stylie777 wrote:

The Init Undef Pass when I rewrote it was designed so that it would only ever support the Architecture's that had the Architecture Specific instructions, with this change, have you checked to see if other architectures could support this? Currently with the changes you have submitted, it would continue to only support existing architectures.

It may be as simple as removing the check to `ST->supportsInitUndef()`, however it may be more involved. At an initial first pass, I cannot say for sure as it has been a while since I looked at this piece of code.

https://github.com/llvm/llvm-project/pull/106744