Jit64: addx - Emit nothing when possible

When the destination register matches a source register, the other
source register contains zero, and overflow isn't needed, the
instruction becomes a nop and we don't need to emit anything.

We could add specialized handling for the case where overflow is needed,
but none of the titles I tried would hit this path.

Before:
83 C7 00             add         edi,0

After:

0 files changed, 0 insertions, 0 deletions