JitBackpatch: fix NOP padding

The new NOP emitter breaks when called with a negative count. As it
turns out, it did happen when deoptimizing 8 bit MOVs because they are
only 4 bytes long and need no BSWAP.
Jit_Util: reduce NOP padding of 8 bit loads

and use MOVSX directly if needed.
Merge pull request #325 from Tilka/nop

Fix NOP padding