[SPIR-V] Add `StorageBufferArrayNonUniformIndexing` extension when accessing `RWStructuredBuffer` with a non-uniform index.

[luciechoi]

Description
Per SPIR-V Specification, StorageBufferArrayNonUniformIndexing capability needs to be added when accessing RWStructuredBuffer with a non-uniform index.

Steps to Reproduce
https://godbolt.org/z/s4PozjYY8

Actual Behavior
OpCapability StorageBufferArrayNonUniformIndexingEXT is missing.

Found while working on llvm/llvm-project#162540.

[luciechoi]

@s-perron