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

[Nathan Gauër via llvm-commits]

Keenuts wrote:

> I believe this should be:
> ; CHECK: %[[#val:]] = OpVectorShuffle %[[#v3]] %[[#load]] %[[#load]] 0 1 2

Good catch, I'll send another PR to fix the load case.
For the store, unless I add a new SPIR-V builtin, I cannot use OpVectorShuffle as the LLVM IR version doesn't allow the 2 vector types to be different (even just in size).
So I'd need to add an intrinsic which is closer to the SPIR-V equivalent.

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