[llvm] [SPIR-V] Add store legalization for ptrcast (PR #135369)

[Nathan Gauër via llvm-commits]

Keenuts wrote:

rebased on main, tests are passing locally (2 previously broken tests have been marked as XFAIL since)
Merging this one CI goes green.

https://github.com/llvm/llvm-project/pull/135369