1. Generalize context specialization such that the provided context
   can be any outer context of the function, not necessarily the
   immediate outer context.

2. Based on this: if function specialization is disabled, then
   specialize for the module context if there is one.

3. Extend typed lowering of module loads and stores such that if
   the operand is a Module constant, we constant-fold the cell load.
   That is, a JSLoadModule with a Module HeapConstant input becomes
   a LoadField with a Cell HeapConstant input, and similarly for
   JSStoreModule.

BUG=v8:1569

Review-Url: https://codereview.chromium.org/2841613002
Cr-Commit-Position: refs/heads/master@{#45083}