[mlir][DialectConversion] Cache type conversions and add a few useful helpers

It is quite common for the same type to be converted many types throughout the conversion process, and there isn't any good reason why we aren't caching that result. Especially given that we currently use identity conversion to signify legality. This revision also adds a few additional helpers to TypeConverter.

Differential Revision: https://reviews.llvm.org/D81679