Remove a dead load and smi untag on all platforms.

runtime/vm/flow_graph_compiler_arm.cc

Index: runtime/vm/flow_graph_compiler_arm.cc
diff --git a/runtime/vm/flow_graph_compiler_arm.cc b/runtime/vm/flow_graph_compiler_arm.cc
index a66bc102c1e716e66f7af0d99502d138b18377ab..4e613061b79c18a8b2bd186f0460abe07fe75737 100644
--- a/runtime/vm/flow_graph_compiler_arm.cc
+++ b/runtime/vm/flow_graph_compiler_arm.cc
@@ -798,9 +798,6 @@ void FlowGraphCompiler::CopyParameters() {
     }
     // Generate code handling each optional parameter in alphabetical order.
     __ ldr(NOTFP, FieldAddress(R4, ArgumentsDescriptor::count_offset()));
-    __ ldr(R6,
-           FieldAddress(R4, ArgumentsDescriptor::positional_count_offset()));
-    __ SmiUntag(R6);
     // Let NOTFP point to the first passed argument, i.e. to
     // fp[kParamEndSlotFromFp + num_args - 0]; num_args (NOTFP) is Smi.
     __ add(NOTFP, FP, Operand(NOTFP, LSL, 1));