[mlir] Fix representation of BF16 constants

This patch is a follow-up on https://reviews.llvm.org/D81127

BF16 constants were represented as 64-bit floating point values due to the lack
of support for BF16 in APFloat. APFloat was recently extended to support
BF16 so this patch is fixing the BF16 constant representation to be 16-bit.

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D81218