[llvm] [CodeGen][Spill2Reg] Initial patch (PR #118832)

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon commented:

I'd like to ensure this patch doesn't get too focussed on just working for gpr->vector spills. In my experience those profitable cases are pretty rare. What has been more useful has been cases such as storing scalars in the upper elements of xmm registers, or even using ymm upper halfs to store xmm vector data, and to a lesser extent storing a i32 in the upper 32-bits of a i64 gpr.

https://github.com/llvm/llvm-project/pull/118832