Codegen: Align representation of stackmap with SpiderMonkey

This commit aligns the representation of stackmaps to be the same
as Spidermonkey's by:
 * Reversing the order of the bitmap from low addresses to high addresses
 * Including incoming stack arguments
 * Excluding outgoing stack arguments

Additionally, some accessor functions were added to allow Spidermonkey
to access the internals of the bitmap.

cranelift/codegen/src/stack_layout.rs

@@ -1,7 +1,7 @@
 //! Computing stack layout.
 
 use crate::ir::stackslot::{StackOffset, StackSize, StackSlotKind};
-use crate::ir::StackSlots;
+use crate::ir::{StackLayoutInfo, StackSlots};
 use crate::result::{CodegenError, CodegenResult};
 use core::cmp::{max, min};
 
@@ -44,6 +44,7 @@ pub fn layout_stack(
     // require the stack to be aligned.
 
     let mut incoming_min = 0;
+    let mut incoming_max = 0;
     let mut outgoing_max = 0;
     let mut min_align = alignment;
     let mut must_align = is_leaf;
@@ -56,6 +57,7 @@ pub fn layout_stack(
         match slot.kind {
             StackSlotKind::IncomingArg => {
                 incoming_min = min(incoming_min, slot.offset.unwrap());
+                incoming_max = max(incoming_max, slot.offset.unwrap() + slot.size as i32);
             }
             StackSlotKind::OutgoingArg => {
                 let offset = slot
@@ -119,8 +121,14 @@ pub fn layout_stack(
         offset &= -(alignment as StackOffset);
     }
 
+    // Set the computed layout information for the frame
     let frame_size = (offset as StackSize).wrapping_neg();
-    frame.frame_size = Some(frame_size);
+    let inbound_args_size = incoming_max as u32;
+    frame.layout_info = Some(StackLayoutInfo {
+        frame_size,
+        inbound_args_size,
+    });
+
     Ok(frame_size)
 }