[mlir][spirv] Erase empty spirv.mlir.loop in LoopPattern

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

Author: CoTinker

mlir/lib/Conversion/SPIRVToLLVM/SPIRVToLLVM.cpp

@@ -1083,6 +1083,12 @@ 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