Jit64: subfic - Optimize constants for d != a

[Sintendo]

These optimizations were already present, but only when d == a. They also make sense when this condition does not hold.

---

imm == 0

Before:

41 BB 00 00 00 00    mov         r11d,0
45 2B DF             sub         r11d,r15d

After:

45 8B DF             mov         r11d,r15d
41 F7 DB             neg         r11d

imm == -1

Before:

41 BD FF FF FF FF    mov         r13d,0FFFFFFFFh
44 2B EE             sub         r13d,esi
0F 93 45 68          setae       byte ptr [rbp+68h]

After:

44 8B EE             mov         r13d,esi
41 F7 D5             not         r13d
C6 45 68 01          mov         byte ptr [rbp+68h],1

[degasus]

Good catch, however I wonder if x64 has an 8bit sign extended immediate mov instruction. This should lead to the same code size and maybe with a lower latency.

[Sintendo]

Unfortunately, there is no such instruction. In fact, of all the x64 'core' instructions, MOV and TEST are the only ones to lack a compact encoding for 8-bit signed immediates.