middle-end: Add MULT_EXPR recognition for cond scalar reduction

gcc/ChangeLog:

	* tree-if-conv.cc (is_cond_scalar_reduction): Add MULT_EXPR
	recognition.

gcc/testsuite/ChangeLog:

	* gcc.dg/tree-ssa/gen-vect-34.c: New test.
	* gcc.dg/vect/vect-ifcvt-18.c: New test.