RakuAST: fix smartmatch on $_ containing a regex boolifying result

Looking at the old compiler frontend, it seems like we don't need the
special smartmatch handling if the right hand side is a plain variable.
While it worked in most cases anyway, things got confusing when that
variable was $_. Then temporarization of that variable disturbed the
check whether the right hand side was a Regex (which doesn't want its
result to be boolified). Avoid the whole thing by just compiling to
a call to infix:sym<~~> for variables. Just like the old frontend.

src/Raku/ast/expressions.rakumod

@@ -219,7 +219,7 @@ class RakuAST::Infix is RakuAST::Infixish is RakuAST::Lookup {
                 nqp::die('Cannot compile bind to ' ~ $left.HOW.name($left));
             }
         }
-        elsif nqp::existskey(OP-SMARTMATCH, $op) {
+        elsif nqp::existskey(OP-SMARTMATCH, $op) && !nqp::istype($right, RakuAST::Var) {
             self.IMPL-SMARTMATCH-QAST($context, $left, $right, nqp::atkey(OP-SMARTMATCH, $op));
         }
         else {