Deleting prologue/epilogue insns

[Richard Kenner]

    As a counterpoint, I often times find the insn helpful, and avoids the
    process of doing -save-temps, and going into the debugger.

In the case an insn not found, I agree, but here it didn't seem as
useful and, as I said, didn't help me one bit.  But if do want to get
into the habit of showing some RTL on an abort, what we should do is
make an abort_with_rtl and start using it in lots of places.