The code generator uses `ip` as a scratch register directly to assemble a
"Constant -> Float stack slot" move. However, the assembler may also use it to
compute the address.

If we try to assemble such a move and the stack slot is out of range of a store
we get the following:
~~~
movw ip, #52429
movt ip, #15820
movw ip, #59328   ; Use ip to compute the address!
movt ip, #65535
str ip, [fp, +ip]
~~~

Bug: 
Change-Id: I97a7b606e3f1d53ed44cc7787e49109cf7a7ab16
Reviewed-on: https://chromium-review.googlesource.com/602230Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Bill Budge <bbudge@chromium.org>
Commit-Queue: Pierre Langlois <pierre.langlois@arm.com>
Cr-Commit-Position: refs/heads/master@{#47370}