aco: Change v_cmp with subgroup invocation to constant.
Based on !20670 (merged)
When a shader has a comparison with the subgroup invocation id, we can use a constant instead, saving a VALU instruction. When the constant can't be represented as a 64-bit literal, use the s_bfm_b64 instruction to generate it instead.
Then, propagate this constant to the branching pseudo instruction for best effect.