New transformation to replace a single-trip loop with an assignment to the loop variable

[arporter]

Given the reshaping of arrays being passed into the UM physics routines, there are many cases where there are loops over a dimension that we know, at build time, will have an extent of just unity. To help the compiler, it would therefore be good to remove all such loops and simply replace them with an assignment to the loop variable followed by the body of the loop:

        if loop.variable.name == "jj":
            # Ensure any symbols that are in the table of the Loop are moved
            # into the outer scope.
            parent_table = loop.parent.scope.symbol_table
            parent_table.merge(loop.loop_body.symbol_table)

            # Move the body of the loop after the loop
            for statement in reversed(loop.loop_body.children):
                loop.parent.addchild(statement.detach(), loop.position + 1)

            # Create the Assignment node
            assign = Assignment.create(Reference(loop.variable),
                                       Literal("1", INTEGER_TYPE))

            # Replace the (now empty) Loop with the Assignment
            loop.replace_with(assign)

Since this isn't entirely trivial, it would be good to have this as a transformation. I'm not sure what it should be called though. Perhaps ReplaceSingleTripLoopTrans?

[arporter]

Since most of the implementation is given in the issue description, this could be a good, self-contained issue for a new person to tackle.

[arporter]

Note, if the body of the loop doesn't contain any CodeBlocks then we could go one better and replace all References to the loop variable with the value supplied to the transformation, i.e. "1" in the example given above.