[IA64] minor performance tune-up in ia64_switch_to

The reenabling of psr.ic should really belong to dtr mapping code block.
It make the fall through code fast since it doesn't need to execute the
predicated-off instruction.  Logically make more sense as well since psr.ic
was turned off in .map code block.

Signed-off-by: Ken Chen <kenneth.w.chen@intel.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>