[llvm] [AMDGPU][DAG] Enable ganging up of memcpy loads/stores for AMDGPU (PR #96185)

[Matt Arsenault via llvm-commits]

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arsenm wrote:

I've never seen "this diff is too large to display before".

These test functions are all kernels, making the pointer arguments all uniform. It would be somewhat more representative to make these regular functions with VGPR pointer arguments. The cases where known uniform pointers is interesting should then use explicit inreg 

https://github.com/llvm/llvm-project/pull/96185