Cleanups in the LLVM IR Dialect

These cleanups reflects some recent changes to the LLVM IR Dialect and the
infrastructure that affects it.  In particular, add documentation on direct and
indirect function calls as well as remove the `call` and `call0` separation.
Change the prefix of custom types from `!llvm.type` to `!llvm` so that it
matches the IR.  Remove the verifier check disallowing conditional branches to
the same block with arguments: identical arguments are now supported, and
different arguments will be caught later.

PiperOrigin-RevId: 237203452