Add a unit test for frame index expansion

[dylanmckay]

Bugzilla Link	39554
Version	trunk
OS	Linux

Extended Description

In avr-rust bug #​112, we encountered a backend bug caused by invalid expansion of a FRMIDX instruction into a MOVWRdRr instruction. The backend would leave an extraneous operand from the original FRMIDX that was not consistent with the definition of MOVWRdRr.

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

More informatiob about that bug can be found at: avr-rust/rust-legacy-fork#112

We should write unit tests that cover the expansion of FRMIDX nodes into MOVWRdRr instructions.