Minor loop-iv tweaks for doloops

[Bernd Schmidt]

This allows us to generate slightly more hardware loops on the Blackfin.
 When walking through arrays on the stack, we often generate comparisons
based on the stack or frame pointer; we should treat these as invariant
everywhere.  Some earlier patches already went in that direction, this
one just converts a few more places.

Bootstrapped and regression tested on i686-linux.  Ok?

I'm also attaching a patch with three Blackfin-specific testcases which
verify that we're generating hardware loops as expected.


Bernd
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