Quick compiler: x86_64 workaround

A recent CL changed the allocation of temp registers destined to
hold a reference such that on 64-bit systems a 64-bit register
would be allocated.  However, for bring-up purposes, the x86_64
backend wants the ability to continue holding long values in a
register pair.

Change-Id: I25d9f755fafbe96144226677f85c6d5cad1ffa76