Add the Briggs optimistic allocator to run on ARM64

Add the Briggs optimistic allocator to run on ARM64
https://bugs.webkit.org/show_bug.cgi?id=168454

Reviewed by Filip Pizlo.

Source/JavaScriptCore:

This patch adds the Briggs allocator to Air:
http://www.cs.utexas.edu/users/mckinley/380C/lecs/briggs-thesis-1992.pdf
It uses it by default on ARM64. I was measuring an 8-10% speedup
in the phase because of this. I also wasn't able to detect a slowdown
for generated code on ARM64. There are still a few things we can do
to speed things up even further. Moving the interference graph into
a BitVector was another 10-20% speedup. We should consider doing this
in a follow up patch. This is especially important now, since making
register allocation faster has a direct impact on startup time for
Wasm modules.

I abstracted away the common bits between Briggs and IRC, and moved
them into a common super class. In a follow up to this patch, I plan
on implementing biased coloring for both Briggs and IRC (this is
described in Briggs's thesis). I was able to detect a 1% slowdown
with Briggs on Octane for x86-64. This is because the register file
for x86-64 is smaller than ARM64. When I implemented biased coloring,
I was no longer able to detect this slowdown. I still think it's a
sensible plan to run Briggs on ARM64 and IRC on x86-64.

* CMakeLists.txt:
* JavaScriptCore.xcodeproj/project.pbxproj:
* b3/air/AirGenerate.cpp:
(JSC::B3::Air::prepareForGeneration):
* b3/air/AirGraphColoring.cpp: Copied from Source/JavaScriptCore/b3/air/AirIteratedRegisterCoalescing.cpp.
(JSC::B3::Air::allocateRegistersByGraphColoring):
(JSC::B3::Air::iteratedRegisterCoalescing): Deleted.
* b3/air/AirGraphColoring.h: Copied from Source/JavaScriptCore/b3/air/AirIteratedRegisterCoalescing.h.
* b3/air/AirIteratedRegisterCoalescing.cpp: Removed.
* b3/air/AirIteratedRegisterCoalescing.h: Removed.
* runtime/Options.h:

Tools:

* Scripts/run-jsc-stress-tests:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@212799 268f45cc-cd09-0410-ab3c-d52691b4dbfc