tree-optimization/125646 - fixup vector types for const mult pattern

When we pattern recognize integer multiplication with a constant
and need a conversion to unsigned we fail to set the appropriate
unsigned vector type on the pattern stmt so it gets the vector
type of the signed result.  This confuses code dealing with UB
for reduction vectorization but will also result in wrong-code
in the non-reduction case.

	PR tree-optimization/125646
	* tree-vect-patterns.cc (vect_synth_mult_by_constant): Assign
	vector type to the pattern def sequence stmts.

	* gcc.dg/vect/pr125646.c: New testcase.

(cherry picked from commit 87039d23cb9ab43f74da905226e1fac9b4de10bc)