For function argument, we used to copy it to a virtual register
if it is referenced outside the entry block. But for an argument
at a fixed frame index, we can use it directly without a virtual
For STATEPOINT's liveness arg, this will make it encode the fixed
stack location directly into the stack map, without spilling to
This slightly improves -O0 code generation (by folding static
stack offset), without losing debuggability.