middle-end: don't form FMAs when multiplication is not single use. [PR108583]

middle-end: don't form FMAs when multiplication is not single use. [PR108583]

The testcase

typedef unsigned int vec __attribute__((vector_size(32)));
vec
f3 (vec a, vec b, vec c)
{
  vec d = a * b;
  return d + ((c + d) >> 1);
}

shows a case where we don't want to form an FMA due to the MUL not being single
use.  In this case to form an FMA we have to redo the MUL as well as we no
longer have it to share.

As such making an FMA here would be a de-optimization.

gcc/ChangeLog:

	PR target/108583
	* tree-ssa-math-opts.cc (convert_mult_to_fma): Inhibit FMA in case not
	single use.

gcc/testsuite/ChangeLog:

	PR target/108583
	* gcc.dg/mla_1.c: New test.

Co-Authored-By: Richard Sandiford <richard.sandiford@arm.com>