Jit64: Merge arithcx into addx/subfx

[Sintendo]

Various optimizations for addx/subfx can be applied to addcx/subfcx as well. Rather than painstakingly replicating these optimizations in arithcx, I feel it makes more sense to remove it entirely and just add handling for the carry flag to addx/subfx.

---

addcx

Optimize for size

Before:

BE 01 00 00 00       mov         esi,1
03 75 A8             add         esi,dword ptr [rbp-58h]

After:

8B 75 A8             mov         esi,dword ptr [rbp-58h]
83 C6 01             add         esi,1

Optimize constant zero

Before:

44 8B C7             mov         r8d,edi
41 83 C0 00          add         r8d,0

After:

44 8B C7             mov         r8d,edi
F8                   clc

---

subfcx

Optimize constant a and b (Example 1)

Before:

BE 00 00 00 00       mov         esi,0
83 EE 08             sub         esi,8

After:

F8                   clc

Optimize constant a and b (Example 2)

Before:

41 BF FF 00 00 00    mov         r15d,0FFh
41 83 EF 08          sub         r15d,8

After:

F9                   stc

Optimize b == 0

Before:

41 B8 00 00 00 00    mov         r8d,0
45 2B C6             sub         r8d,r14d

After:

45 8B C6             mov         r8d,r14d
41 F7 D8             neg         r8d

Optimize constant a

Before:

41 BE 08 00 00 00    mov         r14d,8
41 8B C6             mov         eax,r14d
44 8B F6             mov         r14d,esi
44 2B F0             sub         r14d,eax

After:

44 8B F6             mov         r14d,esi
41 83 EE 08          sub         r14d,8

Optimize a == 0

Before:

44 8B EE             mov         r13d,esi
41 83 ED 00          sub         r13d,0

After:

44 8B EE             mov         r13d,esi
F9                   stc