middle-end: use destination type when folding addent [PR123897]

middle-end: use destination type when folding addent [PR123897]

Previously the types of the 3 operands would always be the same when you get to
convert_mult_to_fma_1.  However now they can differ untill we fold them to be the
same.  Using the type of the final expression is thus incorrect and any
intermediate operations need to happen in the type of the expression being
folded.

gcc/ChangeLog:

	PR tree-optimization/123897
	* tree-ssa-math-opts.cc (convert_mult_to_fma_1): Use type of variable
	being folded.

gcc/testsuite/ChangeLog:

	PR tree-optimization/123897
	* gcc.target/aarch64/sve/pr123897.c: New test.