Reduce use of DecodedInstruction in verifier.

This CL reduces the use of DecodedInstruction in verifier. It continues work
started in https://googleplex-android-review.googlesource.com/#/c/311171.

We cannot remove it completely from verifier because of method
MethodVerifier::VerifyInstruction. It is based on instruction flags rather than
instruction opcode. So we can't rely on instruction format. We may need to add
VRegA, VRegB and VRegC methods for this case. This should also be required for
the compiler itself. Let's delay this change to another CL.

Change-Id: I9d34df8b5bdb3db74e4959364c3cde90ef15585d