aco/optimizer: reduce max alu_opt_info stack operands to 4

ALU instructions typically have a maximum of 3 operands, and even when combining
instructions, the peak count will not go above 4.

Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/38150>

src/amd/compiler/aco_optimizer.cpp

@@ -417,7 +417,7 @@ struct alu_opt_op {
 
 struct alu_opt_info {
    aco::small_vec<Definition, 2> defs;
-   aco::small_vec<alu_opt_op, 5> operands;
+   aco::small_vec<alu_opt_op, 4> operands;
    aco_opcode opcode;
    Format format;
    uint32_t imm;