MIPS: Make invalid LHSs that are calls late errors.

Port r20428 (7a071766)

Original commit message:
Necessary for web legacy compatibility.

Also fold in additional strict mode checks into LHS checks.
Minor constness clean-ups on the way.

BUG=
R=rossberg@chromium.org

Review URL: https://codereview.chromium.org/222893002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20451 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
MIPS: Check that environments assigned via AssignEnvironment are actually used.

Port r20430 (e9457854)

Original commit message:
Check that environments assigned via AssignEnvironment are actually used.

Removed some temporary marker comments on the way.

BUG=
R=plind44@gmail.com

Review URL: https://codereview.chromium.org/214703007

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20452 ce2b1a6d-e550-0410-aec6-3dcde31c8c00