Check constraints for Def operands in the checker (#150)

This was previously skipped: only Use operands were checked to ensure
they had a valid allocation for their constraint.

No new fuzzbugs found, I just noticed this while looking over the
checker.

Author: Amanieu

src/checker.rs

@@ -445,7 +445,7 @@ impl CheckerState {
             return Err(CheckerError::MissingAllocation { inst, op });
         }
 
-        if op.as_fixed_nonallocatable().is_none() {
+        if op.kind() == OperandKind::Use && op.as_fixed_nonallocatable().is_none() {
             match val {
                 CheckerValue::Universe => {
                     return Err(CheckerError::UnknownValueInAllocation { inst, op, alloc });
@@ -506,9 +506,6 @@ impl CheckerState {
                     if !is_here {
                         continue;
                     }
-                    if op.kind() == OperandKind::Def {
-                        continue;
-                    }
 
                     let val = self.get_value(alloc).unwrap_or(&default_val);
                     trace!(