arc: do lr and sr instructions together, avoid instruction pipeline bubble

The lr and sr instructions cause a pipeline bubble. There is an efficiency
to be gained if pairs of lr or sr instructions are done right next to
each other. This can avoid some stall cycles.
Also, r14 and r15 can be used with isa-16 instructions.

Change-Id: I4165365b49da910db31e0699a1a6e47114962942
Signed-off-by: Chuck Jordan <cjordan@synopsys.com>