[llvm] [TableGen][GISel] Create untyped registers during instruction selection (PR #121270)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm commented:

> The type shouldn't matter for GlobalISel; registers created during instruction selection should be virtual (as opposed to generic).

Generic virtual registers are still virtual registers.

I'm not sure the type is entirely useless during selection. computeKnownBits / simplifyDemandedBits will be able to understand a virtual register that still has a type but will give up on a value without one 

https://github.com/llvm/llvm-project/pull/121270