[llvm] Fix stack layout for frames larger than 2gb (PR #84114)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

On non-x86 targets, frame indexes that use an addressing mode that ends up being illegal are not rare.  Rare enough that it doesn't really matter for performance, but not so rare it doesn't come up.  In that case, we scavenge a register and generate equivalent math.  It should be possible to do the same on x86.

There are various tricks you can use to make scavenging happen less frequently; for example, using LocalStackSlotAllocation to lower frame indexes for large allocations.

https://github.com/llvm/llvm-project/pull/84114