[naga] compaction may miss used constants' types

[jimblandy]

I think part of the compaction code is either wrong or redundant.

When compact::compact marks a name constants as used by definition, it also marks its initialization expression as used. But then later in that function, it marks the types of all used constants as used. I'm not sure why these two cases, type and initialization expression, should be handled differently. It seems to me that either the type marking can be moved up (removing that entire loop altogether) or the initializer marking must be moved down, because new constants are getting marked and we'd better include their initialization expressions.

Whatever the case, overrides deserve similar scrutiny.

(I'm not sure this is actually a bug, but there's an asymmetry to naga::compact::compact that seems wrong, and I'm afraid I'll forget about this doubt if I don't write it down.)

[jimblandy]

We have this comment:

    // Constants' initializers are taken care of already, because
    // expression tracing sees through constants. But we still need to
    // note type usage.

I'm not sure it's right.