Fix PR 17368: disable vector mul distribution for square of add/sub for ARM

Generally, it is desirable to distribute (a + b) * c to a*c + b*c for
ARM with VMLx forwarding, where a, b and c are vectors.
However, for (a + b)*(a + b), distribution will result in one extra
instruction.
With distribution:
  x = a + b (add)
  y = a * x (mul)
  z = y + b * y (mla)

Without distribution:
  x = a + b (add)
  z = x * x (mul)

This patch checks if a mul is a square of add/sub. If yes, skip
distribution.

llvm-svn: 191410