[flang] Allow `VariableAssignBufferization` to handle `hlfir::ExprType` (#115136)

Given the following input:
```fortran
1. subroutine ComputeDifferencesKernel
2.   implicit none
3.   integer :: i
4.   integer, dimension(1) :: a
5.
6.   do i = 1, 10
7.     a = [ i ]
8.   end do
9. end subroutine ComputeDifferencesKernel
```
Currently, the assignment in line 7 ends up as a call to the Fortran
runtime function: `AAssign` since the corresponding `hlfir.assign` is
not optimized away by `VariableAssignBufferization`. The reason this
assignment is not optimized away is that `VariableAssignBufferization`
does not match whenever the RHS of the assignment is a `hlfir.expr`
value. However, this behavior is introduced only to prevent clashes
between `VariableAssignBufferization` and `ElementalAssignBufferization`
which optimizes away assignemnts that result from `hlfir.elemental` ops.

This patch relaxes that restriction by checking whether the RHS of an
`hlfir.assign` is the result of `hlfir.elemental` or not. If not, we can
safely proceed with `VariableAssignBufferization`.

Note that in the above example, we won't get a `hlfir.elemental` in the
IR. We would get if we changed line 7 to something like:
```fortran
7.     a = [ i ] + b
```
In which case, `ElementalAssignBufferization` will kick in instead.