[flang] Do not create arith.extui with same from/to type

[clementval]

Summary

In some case the lowering of ichar is generating an arith.extui operation
with the same from/to type. This operation do not accept from/to types to be
the same. If the from/to types are identical, we do not generate the extra
operation.

[jeanPerier]

LGTM

flang/lib/Lower/IntrinsicCall.cpp

if (auto charTy = bufferTy.dyn_cast<fir::CharacterType>()) {
     fir::CharacterType charType =
         fir::CharacterType::get(builder.getContext(), eleType.getFKind(), 1);
     mlir::Type toTy = builder.getRefType(charType);
     mlir::Value cast = builder.createConvert(loc, toTy, buffer);
     charVal = builder.create<fir::LoadOp>(loc, cast);
   }
   LLVM_DEBUG(llvm::dbgs() << "ichar(" << charVal << ")\n");
   auto code = helper.extractCodeFromSingleton(charVal);
+  if (code.getType() == resultType)
+    return code;
   return builder.create<mlir::arith::ExtUIOp>(loc, resultType, code);
 }
 
 // IEOR
 mlir::Value IntrinsicLibrary::genIeor(mlir::Type resultType,
                                       llvm::ArrayRef<mlir::Value> args) {
   assert(args.size() == 2);
   return builder.create<mlir::arith::XOrIOp>(loc, args[0], args[1]);

flang/test/Lower/Intrinsics/ichar.f90

subroutine ichar_test(c)
   print *, ichar(str(J))
 
   ! "Magic" 88 below is the ASCII code for `X` and the value returned by IACHAR (’X’)
   ! CHECK: %[[c88:.*]] = arith.constant 88 : i32
   ! CHECK-NEXT: fir.call @{{.*}}OutputInteger32({{.*}}, %[[c88]])
   ! CHECK-NEXT: fir.call @{{.*}}EndIoStatement
   print *, iachar('X')
 end subroutine
+
+! Check that 'arith.extui' op is not generated if type are matching.
+ ! CHECK-LABEL: no_extui
+subroutine no_extui(ch)
+  integer, parameter :: kind = selected_char_kind('ISO_10646')
+  character(*, kind), intent(in) :: ch(:)
+  integer :: i, j 
+  ! CHECK-NOT: arith.extui
+  j = ichar(ch(i)(i:i))
+end subroutine