[wasm] Add tests for grow_memory in loops

This CL introduces 4 tests that verify that the effects of a grow_memory
instruction executed inside a loop are visible also when the loop is over.

The tests verify the output of the current_memory instruction and the
result of loading a variable stored in the grown memory inside the
loop in the following cases:

 * the memory is grown inside the loop (no memory operation outside);

 * the memory is grown both inside and outside the loop;

R=ahaas@chromium.org,clemensh@chromium.org,gdeepti@chromium.org

Change-Id: I1670aa4d8274f6c54dced98cced7b51534552c36
Reviewed-on: https://chromium-review.googlesource.com/619207Reviewed-by: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Commit-Queue: Enrico Bacis <enricobacis@google.com>
Cr-Commit-Position: refs/heads/master@{#47466}