Modify test, to prevent the next patch breaking it

Modify test, to prevent the next patch breaking it

The upcoming c[lt]z idiom recognition patch eliminates the need for a
brute force computation of the iteration count of these loops. The test
is intended to verify that ivcanon can determine the loop count when the
condition is given by a chain of constant computations.

We replace the constant operations with a more complicated chain that should
resist future idiom recognition.

gcc/testsuite/ChangeLog:

	* gcc.dg/pr77975.c: Make tests more robust.