AMDGPU/SI: Add implicit register operands in the correct order.

This commit fixes a bug in the class 'SIInstrInfo' where the implicit register
machine operands were added to a machine instruction in an incorrect order -
the implicit uses were added before the implicit defs.

I found this bug while working on moving the implicit register operand
verification code from the MIR parser to the machine verifier.

This commit also makes the method 'addImplicitDefUseOperands' in the machine
instruction class public so that it can be reused in the 'SIInstrInfo' class.

Reviewers: Matt Arsenault

Differential Revision: http://reviews.llvm.org/D11689

llvm-svn: 243799