Test stackmap support for floating point types.

Test stackmap support for floating point types.

It appears that float support is complete, or at least, the stackmap records
emitted are not inconceivable (I must admit that I don't know about many of the
architectures under test here).

One curiosity, the SystemZ tests highlight an undocumented (or maybe incorrect)
quirk of the stackmap format: in the case of a Register record, the Offset or
SmallConstant field can encode a sub-register index! I've only ever seen this
field zero for Register entries up until now.