MIPS: Minor cleanup of CompareIC state

Port r13775 (1fd19d83)

Original commit message:
Allow transitioning from KNOWN_OBJECT to KNONW_OBJECT or OBJECT.
Also, rename HEAP_NUMBER IC states to NUMBER, since they handle Smis, too.

BUG=

Review URL: https://codereview.chromium.org/12387076

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13805 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Prepare push to trunk.  Now working on version 3.17.8.

R=mstarzinger@chromium.org
BUG=

Review URL: https://chromiumcodereview.appspot.com/12395012

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13806 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
MIPS: Renamed "symbols" to "internalized strings" throughout the code base, in preparation of the introduction of ES6 'symbols' (aka private/unique names).

Port r13781 (59f39a06)

Original commit message:
The SymbolTable became the StringTable. I also made sure to adapt all comments. The only remaining use of the term "symbol" (other than unrelated uses in the parser and such) is now 'NewSymbol' in the API and the 'V8.KeyedLoadGenericSymbol' counter, changing which might break embedders.

The one functional change in this CL is that I removed the former 'empty_string' constant, since it is redundant given the 'empty_symbol' constant that we also had (and both were used inconsistently).

BUG=

Review URL: https://codereview.chromium.org/12374074

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13807 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Wait for sweeper threads and finalize sweeping only if parallel/concurrent sweeping was in progress.

BUG=

Review URL: https://codereview.chromium.org/12386084

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13810 ce2b1a6d-e550-0410-aec6-3dcde31c8c00