[compiler][amd64] Real mov16 and mov8 added.
Using the movzx instruction, this is for unsigned numbers. Versions
using movsz is needed if signed numbers are to be used.
Inlined a few more opcodes. Fixed branch when (non) zero and branch
when local to correctly treat 0.0 as non-zero.
Fixed clearing of zero type in ADD_LOCAL_INT[_and_pop] and
[compiler] Added ADD_LOCAL_INT to complement ADD_LOCAL_INT_AND_POP.
Also, do quick return for return UNDEFINED.