[PR68097] Try to avoid recursing for floats in gimple_stmt_nonnegative_warnv_p.

It irks me that a PR named "we should track ranges for floating-point
hasn't been closed in this release.  This is an attempt to do just
that.

As mentioned in the PR, even though we track ranges for floats, it has
been suggested that avoiding recursing through SSA defs in
gimple_assign_nonnegative_warnv_p is also a goal.  This patch uses a
global range query (no on-demand lookups, just global ranges and
minimal folding) to determine if the range of a statement is known to
be non-negative.

	PR tree-optimization/68097

gcc/ChangeLog:

	* gimple-fold.cc (gimple_stmt_nonnegative_warnv_p): Call
	range_of_stmt for floats.