tree-optimization/107493 - SCEV analysis with conversions

tree-optimization/107493 - SCEV analysis with conversions

This shows another case where trying to validate conversions during
the CHREC SCC analysis fails because said analysis assumes we are
converting a complete SCC.  Like the constraint on the initial
conversion seen restrict all conversions handled to sign-changes.

	PR tree-optimization/107493
	* tree-scalar-evolution.cc (scev_dfs::follow_ssa_edge_expr):
	Only handle no-op and sign-changing conversions.

	* gcc.dg/torture/pr107493.c: New testcase.