tree-optimization/111445 - simple_iv simplification fault

The following fixes a missed check in the simple_iv attempt
to simplify (signed T)((unsigned T) base + step) where it
allows a truncating inner conversion leading to wrong code.

	PR tree-optimization/111445
	* tree-scalar-evolution.cc (simple_iv_with_niters):
	Add missing check for a sign-conversion.

	* gcc.dg/torture/pr111445.c: New testcase.