Clean up unnecessary bitcasts

[dwightguth]

One thing I noticed while working on #1103 was that there's a lot of places in the code generator where we bitcast one pointer type to another pointer type. This is pretty much redundant now; all pointer types are now the same ptr type, ptr. The bitcasts don't really hurt anything in production because unless you are running on -O0, which we generally don't, the optimization pipeline will remove them. But it's probably still worth cleaning them up wherever possible at some point in order to reduce code complexity.