Split the monster methods in JSNativeContextSpecialization into smaller
ones, adding appropriate helpers. Improve the condition checking for
strings and numbers using CheckString/CheckNumber when applicable. Also
try to merge compatible PropertyAccessInfos, to avoid running into the
polymorphic case whenever possible.

Drive-by-fix: Don't try to resurrect dead nodes during LoadElimination.
With the improve code generation for monomorphic loads, we seem to
trigger the dead node resurrection.

R=epertoso@chromium.org
BUG=v8:4930,v8:5141

Review-Url: https://codereview.chromium.org/2191823002
Cr-Commit-Position: refs/heads/master@{#38127}