[clang] [llvm] Implement WaveGetLaneCount SPIRV lowering (PR #227160)

[Farzon Lotfi via cfe-commits]

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@@ -49,6 +58,22 @@
     }                                                                          \
   }
 
+// 3-arg form: explicit SPIR-V postfix override (perfect for wave->subgroup)
+#define GENERATE_HLSL_INTRINSIC_FUNCTION3(FunctionName, DxilPostfix,           \
+                                          SpirvPostfix)                        \
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farzonl wrote:

This isn't a scalable solution if we need to adopt say a third backed for HLSL.  Are we just going to add a GENERATE_HLSL_INTRINSIC_FUNCTION4? 

Also so much repeated code here between GENERATE_HLSL_INTRINSIC_FUNCTION2 and GENERATE_HLSL_INTRINSIC_FUNCTION3 why couldn't we just make GENERATE_HLSL_INTRINSIC_FUNCTION take 3 args by default?

Or if you are really trying to avoid touching all the `GENERATE_HLSL_INTRINSIC_FUNCTION` Define two macros a GENERATE_HLSL_INTRINSIC_FUNCTION and a GENERATE_HLSL_INTRINSIC_FUNC_BASE. make Base  be  the three arg version  and GENERATE_HLSL_INTRINSIC_FUNCTION be the two arg version. then call the two arg vs 3 arg version indepdendtly.

https://github.com/llvm/llvm-project/pull/227160