[flang][OpenMP] Reassociate ATOMIC update expressions (llvm#153488)

An atomic update expression of form
  x = x + a + b
is technically illegal, since the right-hand side is parsed as (x+a)+b,
and the atomic variable x should be an argument to the top-level +. When
the type of x is integer, the result of (x+a)+b is guaranteed to be the
same as x+(a+b), so instead of reporting an error, the compiler can
treat (x+a)+b as x+(a+b).

This PR implements this kind of reassociation for integral types, and
for the two arithmetic associative/commutative operators: + and *.

Reinstate PR153098 one more time with fixes for the issues that came up:
- unused variable "lsrc",
- use of ‘outer1’ before deduction of ‘auto’.

Author: kparzysz