[mlir][SPIRVToLLVM] Erase empty spirv.mlir.loop in LoopPattern (#113527)

This PR erases `spirv.mlir.loop` with an empty region in `LoopPattern`,
resolving a crash. Fixes #113404.

Author: CoTinker

mlir/lib/Conversion/SPIRVToLLVM/SPIRVToLLVM.cpp

@@ -1148,6 +1148,12 @@ class LoopPattern : public SPIRVToLLVMConversion<spirv::LoopOp> {
     if (loopOp.getLoopControl() != spirv::LoopControl::None)
       return failure();
 
+    // `spirv.mlir.loop` with empty region is redundant and should be erased.
+    if (loopOp.getBody().empty()) {
+      rewriter.eraseOp(loopOp);
+      return success();
+    }
+
     Location loc = loopOp.getLoc();
 
     // Split the current block after `spirv.mlir.loop`. The remaining ops will

mlir/test/Conversion/SPIRVToLLVM/control-flow-ops-to-llvm.mlir

@@ -86,6 +86,14 @@ spirv.module Logical GLSL450 {
 //===----------------------------------------------------------------------===//
 
 spirv.module Logical GLSL450 {
+  // CHECK-LABEL: @empty_loop
+  spirv.func @empty_loop() "None" {
+    // CHECK: llvm.return
+    spirv.mlir.loop {
+    }
+    spirv.Return
+  }
+
   // CHECK-LABEL: @infinite_loop
   spirv.func @infinite_loop(%count : i32) -> () "None" {
     // CHECK:   llvm.br ^[[BB1:.*]]