i#1551 port to ARM: add opnd_create_immed_uint()

To support the many small unsigned immediates on ARM, we add
opnd_create_immed_uint().  For instr creation we recommend just using
pointer-sized immediates with ranges checked at encode time, but for our
own decoding we do set the size and we do want some sanity checks.  Thus we
add opnd_create_immed_uint() which does proper range checks for small
unsigned values.

Review-URL: https://codereview.appspot.com/193810043