scripts/decodecode: make faulting insn ptr more robust

It can accidentally happen that the faulting insn (the exact instruction
bytes) is repeated a little further on in the trace.  This causes that
same instruction to be tagged twice, see example below.

What we want to do, however, is to track back from the end of the whole
disassembly so many lines as the slice which starts with the faulting
instruction is long.  This leads us to the actual faulting instruction
and *then* we tag it.

While we're at it, we can drop the sed "g" flag because we address only
this one line.

Also, if we point to an instruction which changes decoding depending on
the slice being objdumped, like a Jcc insn, for example, we do not even
tag it as a faulting instruction because the instruction decode changes
in the second slice but we use that second format as a regex on the
fsrst disassembled buffer and more often than not that instruction
doesn't match.

Again, simply tag the line which is deduced from the original "<>"
markin...