AArch64: Remove reversedInstructionsWithoutDebug helper

When using reversedInstructionsWithoutDebug to construct a range from a
pair of MachineInstrBundleIterators, the range unexpectedly leaves out an
element. This results in mis-optimization as @mstorsjo points out in
https://reviews.llvm.org/D78157.

The problem is that when we convert a MachineInstrBundleIterator to a
reverse iterator, the result gets incremented:

  MachineInstrBundleIterator(++I.getReverse())

The comment there explains that the "resulting iterator will dereference
... to the previous node, which is somewhat unexpected; but converting
the two endpoints in a range will give the same range in reverse". This
makes it hard to understand what reversedInstructionsWithoutDebug will
do: I've removed the helper to prevent similar mistakes in the future.