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=

Review-Url: https://codereview.chromium.org/2433093002
Cr-Commit-Position: refs/heads/master@{#40862}