bug in loop optimization with -Os (-O2)

[Joern Rennecke]

> I have founded gcc/egcs bug while compile and run test file
> testsuit/gcc.c-torture/execute/loop-2c.c:

The bug has been in biv elimination from the time this optimization
was implemented in gcc.  All the loop2* tests test various mainfestations
of the bug, and the loop3* tests a related bug.  If the bugs are triggered
or not depends on pretty much all the code generation code and previous
optimization passes, in a pseudo-random manner.