[HLSL] Improve logic for choosing spv/dx intrinsic at CG

[sudonatalie]

Follow-up from: #82536

More target-specific intrinsics are going to be added that are effectively shared between SPIR-V and DirectX backends, so we need a better design for HLSL CodeGen than repetitive switch/if statements on the target. There are a few possible patterns discussed here: #82536 (comment).

[llvmbot]

@llvm/issue-subscribers-clang-codegen

Author: Natalie Chouinard (sudonatalie)

Follow-up from: https://github.com//pull/82536

More target-specific intrinsics are going to be added that are effectively shared between SPIR-V and DirectX backends, so we need a better design for HLSL CodeGen than repetitive switch/if statements on the target. There are a few possible patterns discussed here: #82536 (comment).