Fix PR 106900: array-bounds warning inside simplify_builtin_call

Fix PR 106900: array-bounds warning inside simplify_builtin_call

The problem here is that VRP cannot figure out isize could not be 0
due to using integer_zerop. This patch removes the use of integer_zerop
and instead checks for 0 directly after converting the tree to
an unsigned HOST_WIDE_INT. This allows VRP to figure out isize is not 0
and `isize - 1` will always be >= 0.

This patch is just to avoid the warning that GCC could produce sometimes
and does not change any code generation or even VRP.

OK? Bootstrapped and tested on x86_64-linux-gnu with no regressions.

gcc/ChangeLog:

	* tree-ssa-forwprop.cc (simplify_builtin_call): Check
	against 0 instead of calling integer_zerop.