Cache several common LLVMTypes in the LLVMDialect.

LLVM is not thread-safe which means that several of the 'get' methods for LLVMType must be double locked to ensure thread-safety. This cl adds static caching, i.e. no lookups or locking, for several simple LLVM types(i1, half, void, etc.). It also cleans up the implementation of the double locking that is required for some types. In the future we could add a form of dynamic caching to only need to lock one mutex in the best case, but that requires analysis on the memory overhead/vs time lost to taking two locks.

PiperOrigin-RevId: 254806747