[llvm] [GVN] Teach GVN simple masked load/store forwarding (PR #157689)

[Matthew Devereau via llvm-commits]

MDevereau wrote:

I've added an extra test `@forward_masked_load_scalable_type_mismatch` to assert that the types are the same when forwarding. This is to stop an assertion failure when GVN was taking a load & store of a type v4f64, then loading a v4f32 and trying to forward the v4f64 load in its place, which is not valid.

https://github.com/llvm/llvm-project/pull/157689