[InstCombine] fix/enhance fadd/fsub factorization

  (X * Z) + (Y * Z) --> (X + Y) * Z
  (X * Z) - (Y * Z) --> (X - Y) * Z
  (X / Z) + (Y / Z) --> (X + Y) / Z
  (X / Z) - (Y / Z) --> (X - Y) / Z

The existing code that implemented these folds failed to 
optimize vectors, and it transformed code with multiple 
uses when it should not have.

llvm-svn: 339519