Remove duplicate code for searching SharedFunctionInfo.

R=mvstanton@chromium.org

Review URL: https://codereview.chromium.org/762123002

Cr-Commit-Position: refs/heads/master@{#25561}
add fast path for hashing small cons strings

R=yangguo@chromium.org
LOG=N

BUG=437280

Review URL: https://codereview.chromium.org/769453002

Cr-Commit-Position: refs/heads/master@{#25562}