middle-end/123697 - fix .MASK_LOAD_LANES folding

middle-end/123697 - fix .MASK_LOAD_LANES folding

.MASK_LOAD_LANES has an aggregate (array of vectors) return value
which is not compatible with the else value used when trying to
fold this with all lanes inactive.  Instead use an empty CTOR if
the else value is zero and otherwise do not simplify.

	PR middle-end/123697
	* gimple-fold.cc (gimple_fold_partial_load_store): Use an
	empty CTOR for a zero else value in .MASK_LOAD_LANES.

	* gcc.dg/vect/vect-pr123697.c: New testcase.