xtramodes' C_PBIT optimisation transforms:

MOVW          0(R3),R1
ADD           $4,R3,R3

into:

MOVW.P        4(R3),R1

and the AADD optimisation tranforms:

ADD          R0,R1
MOVBU        0(R1),R0

into:

MOVBU        R0<<0(R1),R0

5g does not appear to generate sequences that
can be transformed by xtramodes' AMOVW.

R=remyoudompheng, rsc
CC=golang-dev
https://golang.org/cl/6817085