[ARM][Thumb2InstrInfo] Fix default `0` opcode when rewriting frame indices

The static functions `positiveOffsetOpcode`, `negativeOffsetOpcode` and
`immediateOffsetOpcode` (lib/Target/ARM/Thumb2InstrInfo.cpp) currently can
return `0` as default opcode which is meaningless in this situation.

This patch replaces this default value by llvm_unreachable.

Reviewers: t.p.northover, tellenbach

Reviewed By: tellenbach

Subscribers: tellenbach, kristof.beyls, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D69432

Patch By: Lorenzo Casalino <lorenzo.casalino93@gmail.com>