Quick compiler: fix x86 special identity

The Quick compiler recognizes and specially handles a set of
small methods - including those that simply return one of their
arguments.  For x86, special identity was broken if the returned
argument was a double that was passed half in register and half
in memory.

internal b/17325447

Change-Id: I3a766977cdc0b4216d8ad65a9870c3250b32471e