[HLSL][SPIR-V] Use `OpImageFetch` when loading from `Buffer` type resources

[luciechoi]

https://godbolt.org/z/KKWbanoWv

Currently, OpImageRead is used to read the values from Buffer arrays.

It should be OpImageFetch.

Otherwise, it might not pass the spirv validator.

Discovered while working on #162540

[luciechoi]

@s-perron