tree-optimization/110991 - unroll size estimate after vectorization

tree-optimization/110991 - unroll size estimate after vectorization

The following testcase shows that we are bad at identifying inductions
that will be optimized away after vectorizing them because SCEV doesn't
handle vectorized defs.  The following rolls a simpler identification
of SSA cycles covering a PHI and an assignment with a binary operator
with a constant second operand.

	PR tree-optimization/110991
	* tree-ssa-loop-ivcanon.cc (constant_after_peeling): Handle
	VIEW_CONVERT_EXPR <op>, handle more simple IV-like SSA cycles
	that will end up constant.

	* gcc.dg/tree-ssa/cunroll-16.c: New testcase.