There's no need to have HMapEnumLength as a dedicated instruction,
as it can be expressed using a HLoadNamedField plus an HBitwiseAnd
operation.

R=jarin@chromium.org
BUG=v8:3650
LOG=n

Review URL: https://codereview.chromium.org/1614943002

Cr-Commit-Position: refs/heads/master@{#33439}