Only Heap should be in charge of deciding how to select a subspace for a type

Only Heap should be in charge of deciding how to select a subspace for a type
https://bugs.webkit.org/show_bug.cgi?id=142304

Reviewed by Mark Lam.

This slightly reduces the code duplication for selecting subspace based on type, and what
duplication is left is at least localized in HeapInlines.h. The immediate effect is that
the DFG and FTL don't have to duplicate this pattern.

* dfg/DFGSpeculativeJIT.h:
(JSC::DFG::SpeculativeJIT::emitAllocateJSObject):
(JSC::DFG::SpeculativeJIT::emitAllocateVariableSizedJSObject):
* ftl/FTLLowerDFGToLLVM.cpp:
(JSC::FTL::LowerDFGToLLVM::allocateObject):
* heap/Heap.h:
* heap/HeapInlines.h:
(JSC::Heap::allocateObjectOfType):
(JSC::Heap::subspaceForObjectOfType):
(JSC::Heap::allocatorForObjectOfType):
* runtime/JSCellInlines.h:
(JSC::allocateCell):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@181019 268f45cc-cd09-0410-ab3c-d52691b4dbfc