MIPSr1 doesn't support SEB and SEH instructions and this
causes test InstructionSelectorTest.Word32SarWithWord32Shl to fail.

This CL disables this test on MIPSr1.

TEST=unittests/InstructionSelectorTest.Word32SarWithWord32Shl

Change-Id: I284a85210bd0d38374ca339671643560e8a305e2
Reviewed-on: https://chromium-review.googlesource.com/1164363Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Ivica Bogosavljevic <ibogosavljevic@wavecomp.com>
Cr-Commit-Position: refs/heads/master@{#54939}