[llvm] [SPIRV] Legalize byte-buffer reinterpretation ptrcasts (#192523) (PR #212999)

[Tim Corringham via llvm-commits]

tcorringham wrote:

This change has grown to be rather larger that I originally anticipated. I'm not very familiar with SPIR-V, but this does now allow 8 ByteAddressBuffer offload tests to pass, and also appears to fix the specific failure cases reported in #179560, 192562, and #154669. 

https://github.com/llvm/llvm-project/pull/212999