[turbofan] Relax effects and context for JSToNumber(x:plain-primitive).

Relanded with fix for always returning Change for PlainPrimitive even
if there was no change. The performance regression on primes.js and
corrections.js is due to unlucky loop header alignment; will be addressed
separately.

TEST=unittests
R=svenpanne@chromium.org

Committed: https://chromium.googlesource.com/v8/v8/+/75484e8d16866eba7aa9c3b87841cd6ce2f466b8

Review URL: https://codereview.chromium.org/799413002

Cr-Commit-Position: refs/heads/master@{#25832}
X87: [turbofan] Remove the no-context hack for JSToNumber.

port d211608a3eb7ef3da4d04fd4f5a8540dedbd1faa

original commit message:
  [turbofan] Remove the no-context hack for JSToNumber.

  The ToNumberStub is now able to handle all plain primitives (Numbers,
  Booleans, Null, Undefined and Strings) without context access.

BUG=

Review URL: https://codereview.chromium.org/810683002

Cr-Commit-Position: refs/heads/master@{#25833}