RFA: Fix PR rtl-optimization/20969

[Joern RENNECKE]

This patch goes back to a gcc 3.4 patch, which I have adjusted to work 
in gcc 4.1 .
 
The code patch was bootstrapped & regtested on i686-pc-linux-gnu.
The tm.texi patch was tested with 'make info'.

The matching sh64-elf target hook can be found in the sh-elf-4_1-branch.
The 3.4 version had some elaborate code to gauge the unroll benefit from
the savings on GIV increments, in order to compare it to the cost of 
increasing
the target register pressure, but I can't see any way to do this with 
the existing
4.1 infrastructure, so this part is commented out.
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