This mostly reverts commit 692c254336da71cbe21aaf9fbc21240fd1269b95.
The test "for loop over pointers to struct, getting field from struct
pointer" is still failing on the CI so that one is not moved over.
This reverts commit 0a9b4d092f58595888f9e4be8ef683b2ed8a0da1.
Hm, these are all passing for me locally. I'll have to do some
troubleshooting to figure out which one(s) are failing on the CI.
1. Function signatures that return a no member struct return void
2. Undefined var decls don't get a value generated for them
3. Don't generate bitcast code if the result isn't used, since
bitcast is a pure function. Right now struct handling code
generates some weird unused bitcast AIR, and this optimization
side steps that issue.
