Replace assert! with debug_assert! in production code paths.

This allows the assertions to be disabled in release builds, so that
the code is faster and smaller, at the expense of not performing the
checks. Assertions can be re-enabled in release builds with the
debug-assertions flag in Cargo.toml, as the top-level Cargo.toml
file does.

lib/cretonne/src/ir/stackslot.rs

@@ -244,7 +244,7 @@ impl StackSlots {
     /// Create a stack slot representing an incoming function argument.
     pub fn make_incoming_arg(&mut self, ty: Type, offset: StackOffset) -> StackSlot {
         let mut data = StackSlotData::new(StackSlotKind::IncomingArg, ty.bytes());
-        assert!(offset <= StackOffset::max_value() - data.size as StackOffset);
+        debug_assert!(offset <= StackOffset::max_value() - data.size as StackOffset);
         data.offset = Some(offset);
         self.push(data)
     }
@@ -269,7 +269,7 @@ impl StackSlots {
 
         // No existing slot found. Make one and insert it into `outgoing`.
         let mut data = StackSlotData::new(StackSlotKind::OutgoingArg, size);
-        assert!(offset <= StackOffset::max_value() - size as StackOffset);
+        debug_assert!(offset <= StackOffset::max_value() - size as StackOffset);
         data.offset = Some(offset);
         let ss = self.slots.push(data);
         self.outgoing.insert(inspos, ss);