Implement compareDouble in B3/Air

Implement compareDouble in B3/Air
https://bugs.webkit.org/show_bug.cgi?id=150903

Reviewed by Benjamin Poulain.

A hole in our coverage is that we don't fuse a double comparison into a branch, then we will
crash in the instruction selector. Obviously, we *really* want to fuse double comparisons,
but we can't guarantee that this will always happen.

This also removes all uses of WTF::Dominators verification, since it's extremely slow even in
a release build. This speeds up testb3 with validateGraphAtEachPhase=true by an order of
magnitude.

* assembler/MacroAssembler.h:
(JSC::MacroAssembler::moveDoubleConditionallyFloat):
(JSC::MacroAssembler::compareDouble):
(JSC::MacroAssembler::compareFloat):
(JSC::MacroAssembler::lea):
* b3/B3Dominators.h:
(JSC::B3::Dominators::Dominators):
* b3/B3LowerToAir.cpp:
(JSC::B3::Air::LowerToAir::createCompare):
(JSC::B3::Air::LowerToAir::lower):
* b3/air/AirOpcode.opcodes:
* b3/testb3.cpp:
(JSC::B3::testCompare):
(JSC::B3::testEqualDouble):
(JSC::B3::simpleFunction):
(JSC::B3::run):
* dfg/DFGDominators.h:
(JSC::DFG::Dominators::Dominators):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@194320 268f45cc-cd09-0410-ab3c-d52691b4dbfc