[HLSL] Implement the `smoothstep` intrinsic (#132288)

Closes #99156.


Tasks completed:
- Implement `smoothstep` using HLSL source in `hlsl_intrinsics.h`
- Implement the `smoothstep` SPIR-V target built-in in
`clang/include/clang/Basic/BuiltinsSPIRV.td`
- Add sema checks for `smoothstep` to `CheckSPIRVBuiltinFunctionCall` in
`clang/lib/Sema/SemaSPIRV.cpp`
- Add codegen for spv `smoothstep` to `EmitSPIRVBuiltinExpr` in
`clang/lib/CodeGen/TargetBuiltins/SPIR.cpp`
- Add codegen tests to `clang/test/CodeGenHLSL/builtins/smoothstep.hlsl`
- Add spv codegen test to
`clang/test/CodeGenSPIRV/Builtins/smoothstep.c`
- Add sema tests to
`clang/test/SemaHLSL/BuiltIns/smoothstep-errors.hlsl`
- Add spv sema tests to
`clang/test/SemaSPIRV/BuiltIns/smoothstep-errors.c`
- Create the `int_spv_smoothstep` intrinsic in `IntrinsicsSPIRV.td`
- In SPIRVInstructionSelector.cpp create the `smoothstep` lowering and
map it to `int_spv_smoothstep` in
`SPIRVInstructionSelector::selectIntrinsic`
- Create SPIR-V backend test case in
`llvm/test/CodeGen/SPIRV/hlsl-intrinsics/smoothstep.ll`
- Create SPIR-V backend test case in
`llvm/test/CodeGen/SPIRV/opencl/smoothstep.ll`

1 files changed, 2 insertions, 0 deletions

diff --git a/llvm/lib/Target/SPIRV/SPIRVInstructionSelector.cpp b/llvm/lib/Target/SPIRV/SPIRVInstructionSelector.cpp
index fb37f91..ec2c9d3 100644
--- a/llvm/lib/Target/SPIRV/SPIRVInstructionSelector.cpp
+++ b/llvm/lib/Target/SPIRV/SPIRVInstructionSelector.cpp
@@ -3084,6 +3084,8 @@ bool SPIRVInstructionSelector::selectIntrinsic(Register ResVReg,
     return selectExtInst(ResVReg, ResType, I, CL::rsqrt, GL::InverseSqrt);
   case Intrinsic::spv_sign:
     return selectSign(ResVReg, ResType, I);
+  case Intrinsic::spv_smoothstep:
+    return selectExtInst(ResVReg, ResType, I, CL::smoothstep, GL::SmoothStep);
   case Intrinsic::spv_firstbituhigh: // There is no CL equivalent of FindUMsb
     return selectFirstBitHigh(ResVReg, ResType, I, /*IsSigned=*/false);
   case Intrinsic::spv_firstbitshigh: // There is no CL equivalent of FindSMsb