FP breakage workaround

This should get the x86 runtests passing again until I track down
the root cause.  The bug is related to the invoke/move_result fusing,
but the problem remained after fixing the issue with x86 using a
different return register for floats.  This CL disables the fusing for
x86.

Change-Id: Id2b2ebc5ebd4089fe3053d8f077dcadba3466de0