Function peephole_fuse_load_imm_alu 

pub fn peephole_fuse_load_imm_alu(
    code: &mut [u8],
    bitmask: &mut [u8],
    jump_table: &[u32],
) -> usize

Peephole pass: fuse load_imm(51) + ThreeReg ALU into TwoRegOneImm immediate form.

Scans the PVM code for consecutive pairs where:

1. First instruction is load_imm (opcode 51)
2. Second instruction is a ThreeReg ALU op with an immediate-form equivalent
3. The load destination register equals the ALU output register (dead after ALU)
4. The load value fits in i32 (4-byte immediate)
5. Neither instruction is a branch target

When fusable, rewrites the pair in-place: the first instruction becomes the TwoRegOneImm form with a 4-byte immediate, and all remaining bytes through the end of the second instruction become bitmask=0 continuation bytes.