explain why don't eval array locals as const operands

Author: tesuji

compiler/rustc_mir_transform/src/gvn.rs

@@ -805,7 +805,8 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
             }
             Operand::Copy(ref mut place) | Operand::Move(ref mut place) => {
                 let value = self.simplify_place_value(place, location)?;
-                // In MIR, the array assignments are previously processed before operands.
+                // NOTE(tesuji): We just want to promote rvalues of array kind that assigned to a local, and
+                // not evaluate this local as a const operand of a function to avoid bloating MIR.
                 if let Value::Aggregate(AggregateTy::Array, ..) = self.get(value) {
                     return None;
                 }