Micro-optimize InitPackedArrayLoop

Summary: Its okay to use SIMD registers to move values from the stack to
the array, because PackedArray doesn't use TypedValueAux's m_aux
field. Use the flags from subqi instead of cmp(0,j).

Once those changes were done I noticed an xchg crept into the
code (due to lack of phi-hinting), so fiddled the initialization
order to suppress the xchg.

Reviewed By: @mhahnenberg

Differential Revision: D1632139