rustc_mir_build: Clarify that 'mirrored' does not mean 'flipped' or 'reversed'

My intuition for 'mirrored' is that it means 'flipped' or 'reversed'.
Clarify that that is not what is meant to 'mirror' the THIR from the
HIR.

Author: Enselic

compiler/rustc_mir_build/src/thir/cx/expr.rs

@@ -26,6 +26,8 @@ use crate::thir::cx::Cx;
 use crate::thir::util::UserAnnotatedTyHelpers;
 
 impl<'tcx> Cx<'tcx> {
+    // Create a THIR expression with the same structure as a HIR expression.
+    // (The term "mirror" in this case does not refer to "flipped" or "reversed".)
     pub(crate) fn mirror_expr(&mut self, expr: &'tcx hir::Expr<'tcx>) -> ExprId {
         // `mirror_expr` is recursing very deep. Make sure the stack doesn't overflow.
         ensure_sufficient_stack(|| self.mirror_expr_inner(expr))