[wasm] Add tests for grow_memory in conditional branches

This CL introduces 8 tests that verify that the effects of a grow_memory
instruction executed inside a conditional branch are visible also when
the branch is merged.

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

 * the memory is grown inside the if branch (with no else branch);

 * the memory is grown inside the if branch (when an else branch exists);

 * the memory is grown inside the else branch;

 * the memory is grown by different amount of pages inside both the if
 and else branches.

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

Change-Id: I8127ebdf959eed5b7b5ca5aa6033a7ea41465f32
Reviewed-on: https://chromium-review.googlesource.com/617222Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Enrico Bacis <enricobacis@google.com>
Cr-Commit-Position: refs/heads/master@{#47452}