[flang][NFC] Remove hardcoded attr name for fir.dispatch and fid.dt_entry ops (#81348)

These hardcoded attribute name are a leftover from the upstreaming
period when there was no way to get the attribute name without an
instance of the operation. It is since possible to do without them and
they should be removed to avoid duplication.

This PR cleanup the fir.dt_entry and fir.dispatch ops of these hardcoded
attribute name and use their generated getters. Some other PRs will
follow to cleanup other operations.

Author: clementval

flang/include/flang/Optimizer/Dialect/FIROps.td

@@ -2433,7 +2433,6 @@ def fir_DispatchOp : fir_Op<"dispatch", []> {
     // operand[0] is the object (of class type)
     operand_iterator arg_operand_begin() { return operand_begin() + 1; }
     operand_iterator arg_operand_end() { return operand_end(); }
-    static constexpr llvm::StringRef getMethodAttrNameStr() { return "method"; }
   }];
 }
 
@@ -2922,7 +2921,6 @@ def fir_DTEntryOp : fir_Op<"dt_entry", [HasParent<"TypeInfoOp">]> {
   let hasCustomAssemblyFormat = 1;
 
   let extraClassDeclaration = [{
-    static constexpr llvm::StringRef getMethodAttrNameStr() { return "method"; }
     static constexpr llvm::StringRef getProcAttrNameStr() { return "proc"; }
   }];
 }

flang/lib/Optimizer/Dialect/FIROps.cpp

@@ -2289,12 +2289,11 @@ mlir::ParseResult fir::DTEntryOp::parse(mlir::OpAsmParser &parser,
   // allow `methodName` or `"methodName"`
   if (failed(parser.parseOptionalKeyword(&methodName))) {
     mlir::StringAttr methodAttr;
-    if (parser.parseAttribute(methodAttr,
-                              fir::DTEntryOp::getMethodAttrNameStr(),
+    if (parser.parseAttribute(methodAttr, getMethodAttrName(result.name),
                               result.attributes))
       return mlir::failure();
   } else {
-    result.addAttribute(fir::DTEntryOp::getMethodAttrNameStr(),
+    result.addAttribute(getMethodAttrName(result.name),
                         parser.getBuilder().getStringAttr(methodName));
   }
   mlir::SymbolRefAttr calleeAttr;