Revert of MIPS: Fix bad RegisterConfiguration usage in InstructionSequence unit tests. (patchset #3 id:40001 of https://codereview.chromium.org/2433093002/ )

Reason for revert:
This change rendered InstructionSequenceTest::SetNumRegs ineffectual, thus
loosening the tests that were using that API to ensure correct register
allocation under intentionally constrained setups.

For the problem stated in this CL, a solution needs to continue supporting the
intentionally set-up test configuration.

Original issue's description:
> MIPS: Fix bad RegisterConfiguration usage in InstructionSequence unit tests.
>
> Test InstructionSequenceTest has been initialized with a testing RegisterConfiguration
> instance defined in instruction-sequence-unittest.h, whereas class ExplicitOperand which
> is being tested used RegisterConfiguration from instruction.cc. In case these two
> instances are different, the tests would fail. The issue is fixed by using the same
> instance of RegisterConfiguration both for test code and code under test.
>
> Additionally, the tests in register-allocator-unittest.cc use hardcoded values
> for register and begin failing is the hardcoded register is not available for
> allocation. Fix by forcing the use of allocatable registers only.
>
> TEST=unittests.MoveOptimizerTest.RemovesRedundantExplicit,unittests.RegisterAllocatorTest.SpillPhi
> BUG=
>
> Committed: https://crrev.com/0cf56232209d4c9c669b8426680de18806f6c29a
> Cr-Commit-Position: refs/heads/master@{#40862}

TBR=dcarney@chromium.org,bmeurer@chromium.org,mstarzinger@chromium.org,vogelheim@chromium.org,titzer@chromium.org,ivica.bogosavljevic@imgtec.com
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=

Review-Url: https://codereview.chromium.org/2587593002
Cr-Commit-Position: refs/heads/master@{#41777}