Assign call arguments to stack slots.

When making an outgoing call, some arguments may have to be passed on
the stack. Allocate OutgoingArg stack slots for these arguments and
write them immediately before the outgoing call instruction.

Do the same for incoming function arguments on the stack, but use
IncomingArg stack slots instead. This was previously done in the
spiller, but we move it to the legalizer so it is done at the same time
as outgoing stack arguments.

These stack slot assignments are done in the legalizer before live
range analysis because the outgoing arguments usually are in different
SSSA values with their own short live ranges.

lib/cretonne/src/ir/extfunc.rs

@@ -51,7 +51,7 @@ impl Signature {
         let bytes = self.argument_types
             .iter()
             .filter_map(|arg| match arg.location {
-                            ArgumentLoc::Stack(offset) if offset > 0 => {
+                            ArgumentLoc::Stack(offset) if offset >= 0 => {
                                 Some(offset as u32 + arg.value_type.bytes())
                             }
                             _ => None,