Fix printing of f16 machine operands

Only single and double FP immediates are correctly printed by
MachineInstr::print() during debug output. Half float type goes to
APFloat::convertToDouble() and hits assertion it is not a double
semantics. This diff prints half machine operands correctly.

This cannot currently be hit by any in-tree target.

Patch by Stanislav Mekhanoshin

llvm-svn: 259857