aco/optimizer: Only set scc_needed when it is actually needed.

Not every p_cbranch uses the SCC, but our optimizer thought so.

Fixes: 8a32f57fff
Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7677>

src/amd/compiler/aco_optimizer.cpp

@@ -3235,7 +3235,9 @@ void select_instruction(opt_ctx &ctx, aco_ptr<Instruction>& instr)
    /* Mark SCC needed, so the uniform boolean transformation won't swap the definitions when it isn't beneficial */
    if (instr->format == Format::PSEUDO_BRANCH &&
        instr->operands.size() &&
-       instr->operands[0].isTemp()) {
+       instr->operands[0].isTemp() &&
+       instr->operands[0].isFixed() &&
+       instr->operands[0].physReg() == scc) {
       ctx.info[instr->operands[0].tempId()].set_scc_needed();
       return;
    } else if ((instr->opcode == aco_opcode::s_cselect_b64 ||