VASM part 4: hoist vasm up to cgInst

Summary: Hoist the creation of the main assembler to cgInst; at this point,
each CodeGenerator cg* function must be fully converted to vasm or
left alone (fully x64-asm).

Reviewed By: @alexmalyshev

Differential Revision: D1460398
VASM part 5: decrefs, cgBinaryIntOp, cgBinaryDblOp, less m_rScratch

Summary: Converted the decref family of functions. Instead of emitting code
with an unpatched forward jump, and patching it later, the vasm version
creates the "done" block early, and passes it down to the various decref
helpers as the target to use if there's no decrement needed. (cleaner).

Converted cgCallNative so all CALL_OPCODE opcodes are converted.
vasm-ified cgBinaryIntOp and cgBinaryDblOp, and simplified them somewhat
by de-inlining special cases (noncommutative or overflow-detecting).
(these look more complicated now, but get simpler again in patches
higher in the stack).

Reviewed By: @alexmalyshev

Differential Revision: D1460408