[wasm] Add tests for grow_memory in functions inside loops

This CL introduces 4 test that verify that the effects of a grow_memory
instruction executed in a function invoked inside a loop are visible
also when the loop is over. This is needed because the
AnalyzeLoopAssignment method in function-body-decoder.cc is creating Phi
nodes only for variables assigned inside the loop. The test cases
introduced by this CL verify that the mem_size and mem_start variables
are always correct.

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 in a directly called function inside a loop;

 * the memory is grown in an indirectly called function inside a loop.

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

Change-Id: I2992bf4086b5eac9580c87e2e0ca06364b99714c
Reviewed-on: https://chromium-review.googlesource.com/637911Reviewed-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@{#47674}