[clang] [llvm] [HLSL] Adding HLSL `clip` function. (PR #114588)

[Finn Plummer via cfe-commits]

================
@@ -2120,6 +2123,32 @@ bool SPIRVInstructionSelector::selectSplatVector(Register ResVReg,
   return MIB.constrainAllUses(TII, TRI, RBI);
 }
 
+bool SPIRVInstructionSelector::selectClip(Register ResVReg,
+                                          const SPIRVType *ResType,
+                                          MachineInstr &I) const {
+
+  unsigned Opcode;
+
+  if (STI.isAtLeastSPIRVVer(VersionTuple(1, 6))) {
+    if (!STI.canUseExtension(
+            SPIRV::Extension::SPV_EXT_demote_to_helper_invocation))
+      report_fatal_error(
+          "llvm.spv.clip intrinsic: this instruction requires the following "
+          "SPIR-V extension: SPV_EXT_demote_to_helper_invocation",
+          false);
----------------
inbelic wrote:

```suggestion
  if (STI.isAtLeastSPIRVVer(VersionTuple(1, 6)) ||
      STI.canUseExtension(SPIRV::Extension::SPV_EXT_demote_to_helper_invocation))) {
```

My understanding is that the extension is used to specify the capability is available when we don't have a spirv version that enables it by default. So we can use it if the minimum version is allowed or the extension is enabled.

https://github.com/llvm/llvm-project/pull/114588