The way the code generator's AssembleMove and AssembleSwap methods are written
makes it easy to forget which sort of move is being implemented when looking at
a sequence of instructions. This patch is an attempt to address this by
rewriting those methods using switch/case instead of a string of if/else.

To do this, introduce new utility functions to detect what type of move to
perform given a pair of InstructionOperands.

Bug: 
Change-Id: I32b146c86409e595b7b59a66bf43220899024fdd
Reviewed-on: https://chromium-review.googlesource.com/749201
Commit-Queue: Pierre Langlois <pierre.langlois@arm.com>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50966}