[llvm] Reland "RegisterCoalescer: Add implicit-def of super register when coalescing SUBREG_TO_REG" (PR #168353)

[Jay Foad via llvm-commits]

jayfoad wrote:

> Try to reland #134408, which had to be reverted to a few reported failures.
> 
> The patches [7228ceb](https://github.com/llvm/llvm-project/commit/7228ceb9749d9491710bfa3f872c261942ff15bc) and [9478968](https://github.com/llvm/llvm-project/commit/9478968f2262136f5fe3a738101885de0daef6c7) each fix issues that were reported (with corresponding reproducers/tests). [aebc086](https://github.com/llvm/llvm-project/commit/aebc086b73d15232bfd0c8e8f8099343b28c91a4) was found when enabling subreg liveness tracking for AArch64 (still disabled by default).
> 
> Given the small changes to the original PR, I wasn't sure whether I could just push it, but I figured I'd play it safe instead.

Just a high level comment: since this is now the 8th attempt to land this patch over more than 2 years, would it make sense to restate what this patch does and why? Otherwise you have to follow a very long trail of breadcrumbs to find any kind of explanation.

https://github.com/llvm/llvm-project/pull/168353