The RISC-V hard float calling convention requires the frontend to:
- Detect cases where, once "flattened", a struct can be passed using int+fp or fp+fp registers under the hard float ABI and coerce to the appropriate type(s)
- Track usage of GPRs and FPRs in order to gate the above, and to determine when signext/zeroext attributes must be added to integer scalars
This patch attempts to do this in compliance with the documented ABI, and uses ABIArgInfo::CoerceAndExpand in order to do this. @rjmccall, as author of that code I've tagged you as reviewer for initial feedback on my usage.
Note that a previous version of the ABI indicated that when passing an int+fp struct using a GPR+FPR, the int would need to be sign or zero-extended appropriately. GCC never did this and the ABI was changed, which makes life easier as ABIArgInfo::CoerceAndExpand can't currently handle sign/zero-extension attributes.