The basic intention is to try to remove unnecessary moves caused by
hints in otherwise empty blocks. Roughly:

Before                          After
-----------------------------------------------------------
B0: add   x1, ...               B0: add   x1, ...
    b.ne  B2                        b.eq  B3
B1: mov   x0, x1                B1: [empty]
    b     B3
B2: add   x0, x1, ...           B2: add   x1, x1, ...
B3: phi(B1,B2) in x0            B3: phi(B0,B1) in x1

Hinting is also improved in cases where one of the inputs is already
allocated. This occurs commonly on architectures with instructions which
write into fixed registers, for example.

BUG=

Review-Url: https://chromiumcodereview.appspot.com/2125463002
Cr-Commit-Position: refs/heads/master@{#40498}