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

[via llvm-commits]

qcolombet wrote:

I'm on the fence with that change.

My concerns is that it adds compile time and may trigger other renumbering down the line.
The compile time impact is probably in the noise (@jayfoad have you double check?) and initially at least `getApproxInstrDistance` makes more sense.

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`.

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