tree-optimization/113078 - conditional subtraction reduction vectorization

When if-conversion was changed to use .COND_ADD/SUB for conditional
reduction it was forgotten to update reduction path handling to
canonicalize .COND_SUB to .COND_ADD for vectorizable_reduction
similar to what we do for MINUS_EXPR.  The following adds this
and testcases exercising this at runtime and looking for the
appropriate masked subtraction in the vectorized code on x86.

	PR tree-optimization/113078
	* tree-vect-loop.cc (check_reduction_path): Canonicalize
	.COND_SUB to .COND_ADD.

	* gcc.dg/vect/vect-reduc-cond-sub.c: New testcase.
	* gcc.target/i386/vect-pr113078.c: Likewise.