DuettoBackend: Use by default unsigned arrays for int8 and int16

We don't really care, but that's what WebGL expects
Make duetto.upcast.collapsed fully typed
DuettoBackend: duetto.upcast.collapsed is now totally typed
Fix linker to overwrite opaque types when comparing structs
Dealing with constant gep and bitcast in pointer usage
Merge remote-tracking branch 'upstream/master'
Deal with constants in dfsPointerUsageFlagsComplete + cleanup