The new option is `NoRegisterArgumentsCanComeAfterStackOnes`
If it's set to `true`, then no register argument can follow any stack
argument under the said abi, otherwise such "shuffling" of arguments
is allowed.
This makes the conversions more robust, since everything can rely on
the same tools for filling in missing registers and ordering them based
on the ABI requirements.
This commit introduces a `Kind` for arguments described in
`FunctionType::Layout`. We nowe basically have three types of arguments:
* Scalar: a regular scarlar argument;
* ReferenceToAggregate: a reference to an aggregete argument on the
stack;
* ShadowPointerToAggregateReturnValue: a pointer to the storage for the
(aggregate) return value allocated by the caller;
The architecture-specific pointer (generic register) size is used
instead.
For all the ABIs we support the value of this parameter was already
set to its pointer size. We might need to reintroduce the parameter
in the future if we want to support a super-exotic ABI with different
stack alignment requirements, but that seems highly unlikely.
This also introduces the argument extension helper for one stop all
treating small arguments when they are put into a register or pushed
into the stack.
Split the interface into two functions: `enforce.*` returning the map by
value and `tryApply.*` returning an `std::optional`.
Improve non-positional single deduction function to better reflect
some corner cases (for example how a required `NoOrDead` register
argument is handled).
Enforce a position-based deduction corner case where it's impossible to
deduce whether the argument is passed in the GPR or the vector one -
now GPR takes precedence in those cases.
Ivan Krysak