DFG should be able to inline string equality comparisons

DFG should be able to inline string equality comparisons
https://bugs.webkit.org/show_bug.cgi?id=114224

Source/JavaScriptCore:

Reviewed by Oliver Hunt.

Inline 8-bit string equality, go to slow path for 16-bit strings. 2x speed-up for string equality
comparisons on 8-bit strings. 20-50% speed-up on JSRegress/HashMap tests. 30% speed-up on
string-fasta. 2% speed-up on SunSpider overall. Some small speed-ups elsewhere.

This is a gnarly change but we have loads of test coverage already between the HashMap tests and
preexisting DFG string equality tests (which appear to have been designed to test OSR exits, but
also give us good overall coverage on string equality behavior).

* dfg/DFGFixupPhase.cpp:
(JSC::DFG::FixupPhase::fixupNode):
* dfg/DFGOperations.cpp:
* dfg/DFGOperations.h:
* dfg/DFGSpeculativeJIT.cpp:
(JSC::DFG::SpeculativeJIT::compilePeepHoleBranch):
(JSC::DFG::SpeculativeJIT::compare):
(JSC::DFG::SpeculativeJIT::compileStrictEq):
(JSC::DFG::SpeculativeJIT::compileStringEquality):
(DFG):
* dfg/DFGSpeculativeJIT.h:
(SpeculativeJIT):

LayoutTests:

Reviewed by Oliver Hunt.

* fast/js/regress/script-tests/string-equality.js: Added.
(foo):
* fast/js/regress/string-equality-expected.txt: Added.
* fast/js/regress/string-equality.html: Added.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@147965 268f45cc-cd09-0410-ab3c-d52691b4dbfc