[csa][cleanup] Use the ParameterRepresentation function

See https://cs.chromium.org/chromium/src/v8/src/code-stub-assembler.h?l=244
for the implementation.

Change-Id: I848e86aa69aaa9ceef9fff8b62ad947b0f67b781
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1588235
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Commit-Queue: Jaroslav Sevcik <jarin@chromium.org>
Auto-Submit: Santiago Aboy Solanes <solanes@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61090}

src/code-stub-assembler.cc

@@ -10919,9 +10919,7 @@ Node* CodeStubAssembler::BuildFastLoop(
     ParameterMode parameter_mode, IndexAdvanceMode advance_mode) {
   CSA_SLOW_ASSERT(this, MatchesParameterMode(start_index, parameter_mode));
   CSA_SLOW_ASSERT(this, MatchesParameterMode(end_index, parameter_mode));
-  MachineRepresentation index_rep = (parameter_mode == INTPTR_PARAMETERS)
-                                        ? MachineType::PointerRepresentation()
-                                        : MachineRepresentation::kTaggedSigned;
+  MachineRepresentation index_rep = ParameterRepresentation(parameter_mode);
   VARIABLE(var, index_rep, start_index);
   VariableList vars_copy(vars.begin(), vars.end(), zone());
   vars_copy.push_back(&var);