middle-end: Fix FMA detection when inspecting gimple which have no LHS.

convert_mult_to_fma assumes that all gimple_assigns have a LHS set.  This
assumption is however not true when an IFN is kept around just for the
side-effects.  In those situations you have just the IFN and lhs will be null.

Since there's no LHS, there also can't be any ADD and such it can't be an FMA
so it's correct to just return early if no LHS.

gcc/ChangeLog:

	PR tree-optimization/103253
	* tree-ssa-math-opts.c (convert_mult_to_fma): Check for LHS.

gcc/testsuite/ChangeLog:

	PR tree-optimization/103253
	* gcc.dg/vect/pr103253.c: New test.