[AVR] Fix a backend bug that left extraneous operands after expansion

This patch fixes a bug in the AVR FRMIDX expansion logic.

The expansion would leave a leftover operand from the original FRMIDX,
but now attached to a MOVWRdRr instruction. The MOVWRdRr instruction
did not expect this operand and so LLVM rejected the machine
instruction.

This would trigger an assertion:

    Assertion failed: ((isImpReg || Op.isRegMask() || MCID->isVariadic() ||
                        OpNo < MCID->getNumOperands() || isMetaDataOp) &&
                        "Trying to add an operand to a machine instr that is already done!"),
    function addOperand, file llvm/lib/CodeGen/MachineInstr.cpp

Tim fixed this so that now the FRMIDX is expanded correctly into
a well-formed MOVWRdRr.

Patch by Tim Neumann

llvm-svn: 346117