Also does some bikeshed clean ups. In particualr, the (ARM)
instruction method emitIAS only needs to choose the applicable ARM
instruction, and then passes the corresponding operands to the
corresponding instruction method of the assembler. The assembler
method then extracts the appropriate data from the operands, and
decides which rule to apply for the corresponding arm instruction.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=4334
R=jpp@chromium.org, stichnot@chromium.org

Review URL: https://codereview.chromium.org/1407613002 .