[llvm] [InitUndef] Enable the InitUndef pass on all targets (PR #108353)

[Jay Foad via llvm-commits]

jayfoad wrote:

The AMDGPU usage is a very literal interpretation of what "early clobber" means, i.e. (some part of) the output might get written before (some parts of) the inputs are read.

For the architectural constraints in ARM and RISCV, is the output forbidden from overlapping with any input, or with one specific input? If the latter, maybe a better way to model it would be with something like `Constraints = "$dst != $src2"` in the tablegen definition of the instruction -- but that would require support in all register allocators, and I don't know how much effort anyone wants to put into better handling for this minor codegen issue.

https://github.com/llvm/llvm-project/pull/108353