math-opts: Only build FMA when use is in addends. [PR123940]

math-opts: Only build FMA when use is in addends. [PR123940]

When looking for an FMA opportunity we can find a multiplication use in an
else operand:

  vect_pretmp_50.23_121 = MEM <vector(2) charD.2> [(charD.2 *)&dD.2916 + 14B];
  vect__28.25_123 = vect_pretmp_50.23_121 * { 2, 2 };
  vect_patt_99.26_124 = .COND_ADD ({ -1, -1 }, vect_pretmp_50.23_121, { 14, 15 }, vect__28.25_123);

and build it:
  vect_pretmp_50.23_121 = MEM <vector(2) charD.2> [(charD.2 *)&dD.2916 + 14B];
  vect_patt_99.26_124 = .FMA (vect_pretmp_50.23_121, { 2, 2 }, vect_pretmp_50.23_121);

This patch checks if the use is in one of the addends.

	PR tree-optimization/123940

gcc/ChangeLog:

	* tree-ssa-math-opts.cc (convert_mult_to_fma): Check
	multiplication result is an addend.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/pr123940.c: New test.

Signed-off-by: Robin Dapp <rdapp@oss.qualcomm.com>