[RISCV] Don't merge addi into load/store address if addi has a FrameIndex operand.

This fixes a crash from D124231.

We can't fold
  (load (add base, (addi src, off1)), off2)
     -> (load (add base, src), off1+off2)
if the src is a FrameIndex. FrameIndex cannot be the operand of an
add.

There was an immediate==0 check that I think was trying to catch
the common case of FrameIndex addis where the immediate is 0, but
they can also appear in non-zero form. Instead explicitly check
for a FrameIndex operand.