Promote bf16 to f32 when the target doesn't support it

This is modeled after the half-precision fp support. Two new nodes are
introduced for casting from and to bf16. Since casting from bf16 is a
simple operation I opted to always directly lower it to integer
arithmetic. The other way round is more complicated if you want to
preserve IEEE semantics, so it's handled by a new __truncsfbf2
compiler-rt builtin.

This is of course very bare bones, but sufficient to get a semi-softened
fadd on x86.

Possible future improvements:
 - Targets with bf16 conversion instructions can now make fp_to_bf16 legal
 - The software conversion to bf16 can be replaced by a trivial
   implementation under fast math.

Differential Revision: https://reviews.llvm.org/D126953