[RISCV] Replace untested code with assert

We found untested code where negative frame indices were ostensibly
handled despite it being in a block guarded by !MFI.isFixedObjectIndex.

While the implementation of MachineFrameInfo::isFixedObjectIndex
suggests this is possible (i.e., if a frame index was more negative - less than the
number of fixed objects), I couldn't find any test in tree -- for any
target -- where a negative frame index wasn't also a fixed object
offset. I couldn't find a way of creating such a object with the
public MachineFrameInfo creation APIs. Even
MachineFrameInfo::getObjectIndexBegin starts counting at the negative
number of fixed objects, so such frame indices wouldn't be covered by
loops using the provided begin/end methods.

Given all this, an assert that any object encountered in the block is
non-negative seems reasonable.

Reviewed By: StephenFan, kito-cheng

Differential Revision: https://reviews.llvm.org/D126278