We always know from the context whether a particular id refers to a type
or something else. Split types out into their own map, and add an
accessor which ensures the type actually exists. This isn't so much
about checking for the provided code being valid, and more about
catching likely foul-ups in our own code.

Change-Id: If18831b1b604eed03fbbeaf352272b5ba15b37a9
Reviewed-on: https://swiftshader-review.googlesource.com/c/23608Tested-by: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>