This unblocks moving off having to hold on to a compiled module
template.

Once we don't have the template, when we have a single instance, the
instance and wasm module share the same compiled code. We will want
to clear that code off instance-specific stuff, when the instance is
unreferenced and should be GC-ed (stuff like the instance heap, for
instance). However, the deopt data will maintain a strong reference,
blocking the GC: the module object strongly references the compiled
code, which strongly references the instance object through the deopt
data.

This change addresses that by making that last reference weak.

BUG=v8:5316

Review-Url: https://codereview.chromium.org/2284683005
Cr-Commit-Position: refs/heads/master@{#38990}