6891750: deopt blob kills values in O5
Reviewed-by:

Sometime in 1.5 the deopt blob was rewritten to use I5 to hold a copy
of the current deoptimization mode instead of O5. Before
save_live_registers I5 is actually O5 which might contain a live value
on 64 bit since we rely on register windows to save those values. In
32-bit mode the O registers are explicitly saved since they might
contain 64 bit values. The fix is simply to use an L register as a
cache like we do for G2_thread. I also changed the name from
exception_tmp to deopt_mode since that makes more sense and deleted a
dead declaration.

This was found during JRTS testing of the 64-bit C1 port. Tested with
failing test case.