[mlir][vector] add support for printing f16 and bf16

Love or hate it, but the vector.print operation was the very
first operation that actually made "end-to-end" CHECK integration
testing possible for MLIR. This revision adds support for
the -until recently- less common but important floating-point
types f16 and bf16.

This will become useful for accelerator specific testing (e.g. NVidia GPUs)

Reviewed By: wrengr

Differential Revision: https://reviews.llvm.org/D145207