Small constant addition optimization

[OskarLinde]

It may be a good idea to consider repeated increment/decrement operations for adding small constants to the X and Y registers instead of going through A and an ADC. I've found that for example

y += 2;

Tends to result in expensive register shuffles:

        clc
        tax
        tya
        adc     #2
        tay
        txa

Instead of simply

        iny
        iny

Here's a synthetic example

__attribute((noinline))
uint8_t foo(uint8_t x, uint8_t y) {
    if (x >= 3) { y += 2; x -= 3; }
    return x+y;
}

foo:
        stx     __rc2
        cmp     #3
        bcc     .LBB0_2
        clc
        tax
        lda     __rc2
        adc     #2
        sta     __rc2
        txa
        clc
        adc     #253
.LBB0_2:
        clc
        adc     __rc2
        rts

vs the 10 cycles faster and 7 bytes shorter:

foo:
        cmp     #3
        bcc     .LBB0_2
        inx
        inx
        clc
        adc     #253
.LBB0_2:
        stx     __rc2
        clc
        adc     __rc2
        rts