[llvm] [CodeGen] Renumber slot indexes before register allocation (PR #66334)

[via llvm-commits]

qcolombet wrote:

> > Could you rename `renumberAllIndexes` into `densifyIndices` or something that conveys that we're packing the indices in a contiguous way. Here I'd like to avoid having a name that collide with `renumberIndexes`.
> 
> How about `packIndexes` or `compactIndexes`? (Using "compact" as a verb not a noun, in case that's not obvious.)

Both sounds good to me. Slight preference for `packIndexes`.

https://github.com/llvm/llvm-project/pull/66334