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

(cherry picked from commit e8f74e58)

Change-Id: I9afea5ef34498306528e0804489b5144e9e9aec3