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 that is "the same" as the 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))