[mlir][spirv] Put debug-only variable in LLVM_DEBUG

[antiagainst]

Summary

This avoids paying the cost when building in release.

mlir/lib/Dialect/SPIRV/Transforms/SPIRVConversion.cpp

 template <typename LabelT>
 static LogicalResult checkExtensionRequirements(
     LabelT label, const spirv::TargetEnv &targetEnv,
     const spirv::SPIRVType::ExtensionArrayRefVector &candidates) {
   for (const auto &ors : candidates) {
     if (targetEnv.allows(ors))
       continue;
 
-    SmallVector<StringRef, 4> extStrings;
+    LLVM_DEBUG({
+      SmallVector<StringRef> extStrings;
       for (spirv::Extension ext : ors)
         extStrings.push_back(spirv::stringifyExtension(ext));
 
-    LLVM_DEBUG(llvm::dbgs()
-               << label << " illegal: requires at least one extension in ["
+      llvm::dbgs() << label << " illegal: requires at least one extension in ["
                    << llvm::join(extStrings, ", ")
-               << "] but none allowed in target environment\n");
+                   << "] but none allowed in target environment\n";
+    });
     return failure();
   }
   return success();
 }
 
 /// Checks that `candidates`capability requirements are possible to be satisfied
 /// with the given `isAllowedFn`.
 ///
 ///  `candidates` is a vector of vector for capability requirements following
 /// ((Capability::A OR Capability::B) AND (Capability::C OR Capability::D))
 /// convention.
 template <typename LabelT>
 static LogicalResult checkCapabilityRequirements(
     LabelT label, const spirv::TargetEnv &targetEnv,
     const spirv::SPIRVType::CapabilityArrayRefVector &candidates) {
   for (const auto &ors : candidates) {
     if (targetEnv.allows(ors))
       continue;
 
-    SmallVector<StringRef, 4> capStrings;
+    LLVM_DEBUG({
+      SmallVector<StringRef> capStrings;
       for (spirv::Capability cap : ors)
         capStrings.push_back(spirv::stringifyCapability(cap));
 
-    LLVM_DEBUG(llvm::dbgs()
-               << label << " illegal: requires at least one capability in ["
+      llvm::dbgs() << label << " illegal: requires at least one capability in ["
                    << llvm::join(capStrings, ", ")
-               << "] but none allowed in target environment\n");
+                   << "] but none allowed in target environment\n";
+    });
     return failure();
   }
   return success();
 }
 
 //===----------------------------------------------------------------------===//
 // Type Conversion
 //===----------------------------------------------------------------------===//