loop.c bugfix

[Richard Kenner]

    > I'm somewhat curious -- off the top of my head I thought we wouldn't 
    > generate a base + index address until combine.

    No, strength reduction plays all sorts of games with
    available addressing modes.

True, but irrelevant since the addressing mode in question was there *before*
loop: that's the whole point!

Also, base + index not a valid addressing mode on PA: there is an explicit
pattern for that move insn.