[llvm] [RISCV][GISEL] Legalize G_PTRMASK (PR #73062)

[Thorsten Schütt via llvm-commits]

tschuett wrote:

`G_PTRMASK` is for alias analysis. It should give better results if it exists longer in the pipeline and is only selected at the end. If every target goes through the p->int and int->p early, you will get worse results. Furthermore, GPUs with funny address spaces may do it completely different.

https://github.com/llvm/llvm-project/pull/73062