When an immediate does not fit an add instruction we use a temporary register to
hold the value, using movw/movt to encode it. However, in order to remove a use
of r9 in TurboFan's code generator, we need to cope with no scratch registers
being available. That is to say that the destination and source registers are
the same, and `ip` is not available to use.

In this case, we can split an add instruction into a sequence of additions:
```
UseScratchRegisterScope temps(...);
Register my_scratch = temps.Acquire();
__ add(r0, r0, Operand(0xabcd); // add r0, r0, #0xcd
                                // add r0, r0, #0xab00
```

As a drive-by fix, make the disassembler test fail if we expected a different
number of instructions generated.

Bug: v8:6553
Change-Id: Ib7fcc765d28bccafe39257f47cd73f922c5873bf
Reviewed-on: https://chromium-review.googlesource.com/685014Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Pierre Langlois <pierre.langlois@arm.com>
Cr-Commit-Position: refs/heads/master@{#48491}