Clarify how we deal with "special" registers

Clarify how we deal with "special" registers
https://bugs.webkit.org/show_bug.cgi?id=129806

Already reviewed change being relanded.

Relanding change set r165196 as it wasn't responsible for the breakage reported in
https://bugs.webkit.org/show_bug.cgi?id=129822.  That appears to be a build or

Reviewed by Michael Saboff.
configuration issue.

* assembler/ARM64Assembler.h:
(JSC::ARM64Assembler::lastRegister):
* assembler/MacroAssembler.h:
(JSC::MacroAssembler::nextRegister):
* ftl/FTLLocation.cpp:
(JSC::FTL::Location::restoreInto):
* ftl/FTLSaveRestore.cpp:
(JSC::FTL::saveAllRegisters):
(JSC::FTL::restoreAllRegisters):
* ftl/FTLSlowPathCall.cpp:
* jit/RegisterSet.cpp:
(JSC::RegisterSet::reservedHardwareRegisters):
(JSC::RegisterSet::runtimeRegisters):
(JSC::RegisterSet::specialRegisters):
(JSC::RegisterSet::calleeSaveRegisters):
* jit/RegisterSet.h:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@165293 268f45cc-cd09-0410-ab3c-d52691b4dbfc