Enforce that arrays in expressions are ArrayVars

[ivogabe]

Description

This should be reviewed and merged after #455!

This PR enforces that array computations nested in expressions are always an ArrayVar. Previously this was practically the case, as we assumed that it only contained variables. We enforce this by removing the acc type argument of PreOpenExp. This also means that the expressions don't have to be compiled from for instance PreOpenExp OpenAcc to PreOpenExp DelayedAcc.

Motivation and context

How has this been tested?

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.