Fix JIT bug related to immediate doubles

Fix JIT bug related to immediate doubles

Change 256211 (JIT: Performance Fix for const doubles) introduced a
defect that can cause the JIT to use the wrong floating point
double constant in traces in which the following conditions hold:

   o Two (or more) different 64-bit floating point constants are used.
   o The physical register holding the first constant is still live
     at the time the second constant is used.
   o The low 32 bits of the two constants are identical.

In this situation, the load/copy optimization pass will incorrectly
determine that the two constants are the same, delete the load of
the second constant and re-use the first constant value.

Note: this problem only occurs with 64-bit floating point literals.
64-bit long literals are unaffected.

This CL works around the problem, and a subsequent CL will rework
disambiguation of 64-bit immediates in a somewhat cleaner fashion.

Change-Id: I33baf78402bab58d9b0ca46189f26491c2b2a751