[mlir] Migrate away from TypeRange(std::nullopt) (NFC) (#145246)

ArrayRef has a constructor that accepts std::nullopt.  This
constructor dates back to the days when we still had llvm::Optional.

Since the use of std::nullopt outside the context of std::optional is
kind of abuse and not intuitive to new comers, I would like to move
away from the constructor and eventually remove it.

One of the uses of std::nullopt is in a the constructors for
TypeRange.  This patch takes care of the migration where we need
TypeRange() to facilitate perfect forwarding.  Note that {} would be
ambiguous for perfecting forwarding to work.

mlir/lib/Conversion/SCFToGPU/SCFToGPU.cpp

@@ -245,7 +245,7 @@ void AffineLoopToGpuConverter::createLaunch(AffineForOp rootForOp,
   Location terminatorLoc = terminator.getLoc();
   terminator.erase();
   builder.setInsertionPointToEnd(innermostForOp.getBody());
-  builder.create<gpu::TerminatorOp>(terminatorLoc, std::nullopt);
+  builder.create<gpu::TerminatorOp>(terminatorLoc, TypeRange());
   launchOp.getBody().front().getOperations().splice(
       launchOp.getBody().front().begin(),
       innermostForOp.getBody()->getOperations());

mlir/lib/Conversion/SPIRVToLLVM/SPIRVToLLVM.cpp

@@ -839,7 +839,7 @@ public:
                   ConversionPatternRewriter &rewriter) const override {
     if (callOp.getNumResults() == 0) {
       auto newOp = rewriter.replaceOpWithNewOp<LLVM::CallOp>(
-          callOp, std::nullopt, adaptor.getOperands(), callOp->getAttrs());
+          callOp, TypeRange(), adaptor.getOperands(), callOp->getAttrs());
       newOp.getProperties().operandSegmentSizes = {
           static_cast<int32_t>(adaptor.getOperands().size()), 0};
       newOp.getProperties().op_bundle_sizes = rewriter.getDenseI32ArrayAttr({});

mlir/test/lib/Dialect/Test/TestPatterns.cpp

@@ -1008,7 +1008,7 @@ struct TestPassthroughInvalidOp : public ConversionPattern {
                                   op->getOperand(it.index()).getType(), range)
               .getResult());
     }
-    rewriter.replaceOpWithNewOp<TestValidOp>(op, std::nullopt, flattened,
+    rewriter.replaceOpWithNewOp<TestValidOp>(op, TypeRange(), flattened,
                                              std::nullopt);
     return success();
   }
@@ -1024,7 +1024,7 @@ struct TestDropAndReplaceInvalidOp : public ConversionPattern {
   LogicalResult
   matchAndRewrite(Operation *op, ArrayRef<Value> operands,
                   ConversionPatternRewriter &rewriter) const final {
-    rewriter.replaceOpWithNewOp<TestValidOp>(op, std::nullopt, ValueRange(),
+    rewriter.replaceOpWithNewOp<TestValidOp>(op, TypeRange(), ValueRange(),
                                              std::nullopt);
     return success();
   }