Adds basic assignment instructions, mov, movn, movw, movt,
ldr, etc. in order to copy around the first few integer
(i32, i64) arguments out of r0 - r3, and then return then.

The "mov" instruction is a bit special and can actually
be a "str" when the dest is a stack slot.

Model the Memory operand types, and the "flexible Operand2".
Add a few tests demonstrating the flexibility of the
immediate encoding.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=4076
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/1127963004