Speed up ExpressionClassifier::Accumulate

The following changeset introduced a regression on CodeLoad of about 3%:

  https://chromium.googlesource.com/v8/v8/+/e73594c7fb3e6b5834b7ddfe78727fb994bab25f

This CL should reduce the regression to 1.5%.

R=dslomov@chromium.org
LOG=N
BUG=

Review URL: https://codereview.chromium.org/1163323006

Cr-Commit-Position: refs/heads/master@{#28872}