aco: don't rematerialize exec

Since exec is not considered a temporary anymore,
we accidentally allowed to rematerialize it.

Fixes: a56ddca4e8 ('aco: make all exec accesses non-temporaries')
Closes: #4327

Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9215>

src/amd/compiler/aco_spill.cpp

@@ -250,7 +250,7 @@ bool should_rematerialize(aco_ptr<Instruction>& instr)
 
    for (const Operand& op : instr->operands) {
       /* TODO: rematerialization using temporaries isn't yet supported */
-      if (op.isTemp())
+      if (!op.isConstant())
          return false;
    }