perf(parser): short-circuit _parse_pivots when next token isn't PIVOT/UNPIVOT (#7557)

_parse_pivots runs once per Join and most table refs, but always
builds an iter+list even when the next token can never start a PIVOT
clause. Guard on the token type up front so the common no-pivot path
is a single check.

Small but consistent win under mypyc (~1-3% on join-heavy queries
like many_joins). No behavior change.

Author: tobymao

sqlglot/parser.py

@@ -5010,6 +5010,8 @@ def _parse_table_sample(self, as_modifier: bool = False) -> exp.TableSample | No
         )
 
     def _parse_pivots(self) -> list[exp.Pivot] | None:
+        if self._curr.token_type not in (TokenType.PIVOT, TokenType.UNPIVOT):
+            return None
         return list(iter(self._parse_pivot, None)) or None
 
     def _parse_joins(self) -> t.Iterator[exp.Join]: