https://bugs.webkit.org/show_bug.cgi?id=125771

Reviewed by Geoffrey Garen.

Source/WebCore: 

Add a brain dead register allocator to simplify the use of registers
when writting code generators.

RegisterAllocator has two purposes:
-make it easy to name registers properly.
-make it hard to reuse a register accidentally.

A helper class LocalRegister is also defined to provide an easy
way to work with registers within a C++ scope. For example:
    LocalRegister elementPointer(allocator);
    assembler.load(address, elementPointer);

RegisterAllocator makes no attempt at optimizing register allocations, but it reduces
implicit dependencies by returning used register at the end of the queue, making it less
likely they will be reused before their last instruction is executed.

The current implementation only support unix x86_64, it only uses caller saved registers.

* WebCore.xcodeproj/project.pbxproj:
* cssjit/RegisterAllocator.h: Added.
(WebCore::RegisterAllocator::allocateRegister): Provides any available register.
To restrict runtime exploitation of compiler bugs, the method crashes in release
if the register pool is empty.

(WebCore::RegisterAllocator::reserveRegister): Reserve a particular register.
(WebCore::RegisterAllocator::returnRegister): Return a previously allocated or reserved register.

(WebCore::RegisterAllocator::allocatedRegisters):
(WebCore::LocalRegister::LocalRegister):
(WebCore::LocalRegister::~LocalRegister):
(WebCore::LocalRegister::operator JSC::MacroAssembler::RegisterID):
(WebCore::RegisterAllocator::RegisterAllocator):

Source/WTF: 

* wtf/Platform.h: Add a new flag "CSS_SELECTOR_JIT" to guard
an experimental JIT compiler in WebCore.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@160697 268f45cc-cd09-0410-ab3c-d52691b4dbfc