[llvm] [SPIR-V] Support `nonuniformindex` intrsinsic in SPIRV CodeGen. (PR #162540)

[Steven Perron via llvm-commits]

s-perron wrote:

> But this feature seem to be performance-related, so technically we can reuse one of these mechanisms.

This is related to correctness. If the decoration and corresponding capability is not present, Vulkan drivers will assume indexing is uniform and simply take the value in the first thread when indexing the resource array. This is incorrect behaviour. It is annoying that SPIR-V  defaulted to the fast behaviour instead of the conservative behaviour. The mechanism we use has to be preserved through optimization.


https://github.com/llvm/llvm-project/pull/162540