Add directize-skip-type-mismatch pass argument

[thiblahute]

When fpcast-emu will run later in the pipeline, directize should not replace type-mismatched call_indirect with unreachable. The new --pass-arg=directize-skip-type-mismatch tells directize to return Unknown instead of Trap for these cases, leaving the call_indirect intact for fpcast-emu to handle.

Without this, emscripten's pass pipeline (-O2 then fpcast-emu) causes directize to kill type-mismatched calls before fpcast-emu can fix them, resulting in RuntimeError: unreachable at runtime. This affects any C code using -sEMULATE_FUNCTION_POINTER_CASTS that calls functions through cast function pointers (e.g. GLib's g_list_free_full casting GDestroyNotify to GFunc).

[kripken]

Interesting problem, thanks for the PR!

Thinking about this, two other options might be

• The user can disable Directize entirely, if they know they need this kind of fixup, using --skip-pass=directize. That would avoid adding a flag for this rare case, and the cost of disabling the entire pass might be low?
• The user can reorder the operations, first letting fpcast-emu handle these calls, and then running the optimizer and Directize. I'm not actually sure how this would affect performance. (edit: I mean that this might actually help)

In both cases we might need to measure the impact.

Thoughts?