MIPS: Turn the NumberToStringStub into a hydrogen stub.

Port r16910 (5a0dd952)

Original commit message:
This adds a BuildLookupNumberStringCache() in Hydrogen, which will
be used by the hydrogen version of StringAddStub, in addition to the
hydrogen version of NumberToStringStub.

BUG=
R=plind44@gmail.com

Review URL: https://codereview.chromium.org/24436004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16926 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
MIPS: replace RegExpCEntryStub with DirectCEntryStub.

Port r16618 (0eebc593)

Original commit message:
RegExpCEntryStub is therefore removed.

BUG=none
TEST=none
R=plind44@gmail.com

Review URL: https://codereview.chromium.org/24321002

Patch from Balazs Kilvady <kilvadyb@homejinni.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16927 ce2b1a6d-e550-0410-aec6-3dcde31c8c00