passes: opt_share: don't statically initialize mergeable_type_map

passes: opt_share: don't statically initialize mergeable_type_map

In 3d3779b0376b8204ed7637053176a07b7271ac1d this got turned from a
`std::map<std::string, std::string>` to `std::map<IdString, IdString>`.
Consequently, this exposed some initialization sequencing issues (#1361).

Only initialize the map when it's first used, to avoid these static issues.

This fixes #1361.

Signed-off-by: Sean Cross <sean@xobs.io>