loop-iv bugfix

[Bernd Schmidt]

This fixes a bug I discovered while working on the previous patch to 
improve upper bounds for the number of iterations.  When searching 
backwards through the insns to simplify an expression, loop-iv keeps 
track of which registers have been altered, but it does not reliably 
take this information into account.  Whenever a register that appears in 
the expression has been altered, we must abort the scan.

Bootstrapped & regression tested on i686-linux, as well as numerous 
times in our Blackfin tree.  Committed as 122291.


Bernd

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