[hlsl] more matCx2 fixes

[teoxoy]

Connections
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Description
See commits.

Testing
Added more snapshots.

Squash or Rebase?
Rebase

[andyleiserson]

It's not a new issue with this change, and it doesn't need to block merging, but it looks like we're missing the reserved names for this stuff in keywords.rs.

This might be worth a CHANGELOG mention.

[teoxoy]

WGSL and GLSL (our two text frontends) don't allow identifiers to start with __ so I thought that's ok but I guess it would be clearer to add them to the namer?

[andyleiserson]

WGSL and GLSL (our two text frontends) don't allow identifiers to start with __ so I thought that's ok but I guess it would be clearer to add them to the namer?

I'm not sure if it's a small or large portion of the instances that exist, but there are at least some naga-defined __ identifiers in the keywords file. It does seem useful to have an inventory somewhere of names that naga uses, even if we don't explicitly check for conflicts with user identifiers. Possibly we should reject __ in the validator as well? I'll file an issue.