For handling restartability issues, following conditions must be
fulfilled:
- For Branch-and-link restartability: GPR 31 (ra register) must not be
used for the source register rs.
- For Jump-and-Link restartability: Register specifiers rs and rd must
not be equal.
This CL implements checking that GPR 31 is not used as source register.

TEST=
BUG=

Change-Id: I568ff9c497b4efca73f1a5353cb7520202524479
Reviewed-on: https://chromium-review.googlesource.com/549362Reviewed-by: Ivica Bogosavljevic <ivica.bogosavljevic@imgtec.com>
Commit-Queue: Ivica Bogosavljevic <ivica.bogosavljevic@imgtec.com>
Cr-Commit-Position: refs/heads/master@{#46252}