Further improve the ctz recognition: Avoid ICEing on negative shift
counts or multiply constants.  Check the type is a char type for the
string constant case to avoid accidentally matching a wide STRING_CST.
Add a tree_expr_nonzero_p check to allow the optimization even if
CTZ_DEFINED_VALUE_AT_ZERO returns 0 or 1.  Add extra test cases.

Bootstrap OK on AArch64 and x64.

    gcc/
	PR tree-optimization/93231
	* tree-ssa-forwprop.c (optimize_count_trailing_zeroes): Check
	input_type is unsigned.  Use tree_to_shwi for shift constant.
	Check CST_STRING element size is CHAR_TYPE_SIZE bits.
	(simplify_count_trailing_zeroes): Add test to handle known non-zero
	inputs more efficiently.

    testsuite/
	PR tree-optimization/93231
	* gcc.dg/pr90838.c: New test.
	* gcc.dg/pr93231.c: New test.
	* gcc.target/aarch64/pr90838.c: Use #define u 0.