Adds word-size store instructions to x64 assembler

Formerly, trying to emit a 16-bit instruction, such as
with test_imm16_disp_reg64(...) would actually emit a testl
instead of a testw. This would emit an 8-byte instruction
(ex. 41 f7 45 0e 01 01 00 00) and possibly be testing the wrong
thing. Now, when you emit a word-size instruction, it will prefix
with 66 and use the correct operand size, saving us a byte
(ex. 66 41 f7 45 0e 01 01).

I also converted the instruction emission in ContPreNext to
use the new style assembly emission.