[flang][x86] transform mulss to xorps for sign(1.,x) * sign(1.,y)

[vfdff]

• test: https://gcc.godbolt.org/z/zcxKG63E3

  integer function test_sign(x, y)
  implicit none
  real, intent(in) :: x, y
  print *, sign(1.,x) * sign(1.,y)
  end function test_sign

• gfortran: xorps xmm0, xmm1 (line 26)

• flang-new: mulss %xmm1, %xmm0 (line 28)

[vfdff]

• C++ test shows that the clang also has similar issue , https://gcc.godbolt.org/z/jss3fcKva