[clang] [HLSL] Implement HLSL splatting (PR #118992)

[Chris B via cfe-commits]

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@@ -370,6 +370,9 @@ CAST_OPERATION(HLSLArrayRValue)
 // Aggregate by Value cast (HLSL only).
 CAST_OPERATION(HLSLElementwiseCast)
 
+// Splat cast for Aggregates (HLSL only).
+CAST_OPERATION(HLSLSplatCast)
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llvm-beanz wrote:

nit: can we call this `HLSLAggregateSplatCast` I know it is a bit mouthy, but it clarifies the difference between this and the `VectorSplat` cast that already exists.

https://github.com/llvm/llvm-project/pull/118992