Should parse `x = def f = p 1`

[qnighy]

Found during investigation of ruby#21153, Prism cannot parse this:

% ./miniruby --parser=parse.y -e "x = def f = p 1"
% ./miniruby --parser=prism -e "x = def f = p 1"
-e: -e:1: syntax error found (SyntaxError)
> 1 | x = def f = p 1
    |               ^ unexpected integer, expecting end-of-input