[AArch64] Remove superfluous sxtw in peephole opt (#96293)
Across a basic-block we might have an i32 extract from a value that only
operates on upper bits (for example a sxtw). We can replace the COPY
with a new version skipping the sxtw.
This is a re-commit of 7f2a5dfe35f8bbaca2819644c7aa844f938befd6, with a fix for
removing all the intermediate COPY nodes (and some extra debug logging).
