[mlir][Type] Remove the remaining usages of Type::getKind in preparation for its removal

This revision removes all of the lingering usages of Type::getKind. A consequence of this is that FloatType is now split into 4 derived types that represent each of the possible float types(BFloat16Type, Float16Type, Float32Type, and Float64Type). Other than this split, this revision is NFC.

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D85566