rustc_span: Optimize syntax context comparisons

Including comparisons with root context

compiler/rustc_span/src/source_map/tests.rs

@@ -18,7 +18,7 @@ impl SourceMap {
     ///    * the LHS span must start at or before the RHS span.
     fn merge_spans(&self, sp_lhs: Span, sp_rhs: Span) -> Option<Span> {
         // Ensure we're at the same expansion ID.
-        if sp_lhs.ctxt() != sp_rhs.ctxt() {
+        if !sp_lhs.eq_ctxt(sp_rhs) {
             return None;
         }