tree-optimization/26854 - compile-time hog in SSA forwprop

The following addresses

 tree forward propagate             :  12.41 (  9%)

seen with the compile.i testcase of this PR which points at
the has_use_on_stmt function which, for SSA names with many
uses is slow.  The solution is to instead of immediate uses,
look at stmt operands to identify whether a name has a use
on a stmt.  That improves SSA forwprop to

 tree forward propagate             :   1.30 (  0%)

for this testcase.

	PR tree-optimization/26854
	* gimple-fold.cc (has_use_on_stmt): Look at stmt operands
	instead of immediate uses.