Add LSL (register, immediate) to ARM integrated assembler.

Also does some clean up on emitType01 methods (making optional argument
explicit, and moving rule checks to the lowest level).

BUG= https://bugs.chromium.org/p/nativeclient/issues/detail?id=4334
R=jpp@chromium.org

Review URL: https://codereview.chromium.org/1456783003 .