[SPIR-V] `fma` emits `GLSL.std.450 Fma` instead of `OpFmaKHR`

Description
When compiling the HLSL fma intrinsic to SPIR-V, DXC emits the GLSL.std.450 Fma instruction. According to the Vulkan spec, this instruction is not guaranteed to be implemented in hardware as a fused operation. HLSL's fma intrinsic specifically requires fused semantics, so emitting this GLSL instruction does not seem correct. This also appears to be the root cause of this issue in the offload-test-suite. On Intel Vulkan, the driver gives the non-fused output for one of the precision sensitive constant-folding cases (though the buffer equivalent case produces the fused/correct result).

The fix for this would be to emit OpFmaKHR from SPV_KHR_fma instead, which guarantees a fused operation. This is also what Clang currently does.

Repro
https://godbolt.org/z/ezEexaGMa

Start with the Godbolt repro and trace how HLSL’s fma intrinsic is lowered to SPIR-V, comparing the current GLSL.std.450 Fma path with Clang’s existing handling. The fix is complete when the generated SPIR-V uses OpFmaKHR from SPV_KHR_fma and the precision-sensitive repro produces fused results.