[ppc] Partial fix for 16458

[Nathan Sidwell]

This is a partial fix for 16458, where we generated a signed compare
for equality followed by an unsigned compare for ordering.  When expanding
the first compare, we should generate an unsigned compare if the operands
are unsigned, so that we have a chance of CSEing a later compare.
Unfortunately we can only do this with subregs, which provide the signedness
information.

booted & tested on powerpc64-unknown-linux-gnu, ok?

nathan

-- 
Nathan Sidwell    ::   http://www.codesourcery.com   ::     CodeSourcery LLC
nathan@codesourcery.com    ::     http://www.planetfall.pwp.blueyonder.co.uk

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