gh-153568: Factor shared prefixes in expression rules - #157437

Several expression rules start by parsing the same thing for each choice. For a - b, the parser tries the + alternative first, then goes back and gets a again to try -. The cache saves us from parsing a from scratch, but we still pay for the extra rule call and cache lookup.

This puts the shared part before the choice: get the left operand once, then check which operator follows. It does the same for tuples, boolean expressions, comparisons and powers. Parse the first expression once, then look for the optional part that follows it. When there is nothing more to match, return the expression we already have.

On my machine, this makes parsing the full standard library and ten repositories about 4.7% faster overall.

The table compares this change against main at a60343ed17, using release builds without PGO or LTO. The timings include tokenization, building the AST and cleanup, without compiling bytecode or running the files.

Part of #153568.

• Issue: Speed up the parser #153568