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.

(cherry-pick of c1757a6deab0ca0bfd42c38612d92b2f26c41dbe.)

Change-Id: I795b4b753550d2745cbbdd83ae25f4a7088990f6