Jit64: subfx - Special case b == 0

Happens in Super Mario Sunshine. You could probably do something similar
for b == -1 (like we do for subfic), but I couldn't find any titles that
do this.

- Case 1: d == a

Before:
41 8B C7             mov         eax,r15d
41 BF 00 00 00 00    mov         r15d,0
44 2B F8             sub         r15d,eax

After:
41 F7 DF             neg         r15d

- Case 2: d != a

Before:
BF 00 00 00 00       mov         edi,0
41 2B FD             sub         edi,r13d

After:
41 8B FD             mov         edi,r13d
F7 DF                neg         edi

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