Specifically an attempt to address a 3.5% regression on the total load
time on cnn introduced by https://codereview.chromium.org/2113673002.

Non-refactoring effect of this CL is to reduce the number of branches in
CodeStubAssembler-generated loops iterating over FixedArrays from
two to one.

LOG=N
BUG=v8:5423

Review-Url: https://codereview.chromium.org/2380953002
Cr-Commit-Position: refs/heads/master@{#40020}