[CS] Fix locator for `if` expressions
Currently we don't insert an intermediate
`SyntacticElement` locator element for the `if`
statement in a SingleValueStmtExpr, which leads
to an assertion failure when attempting to simplify
a following `TernaryBranch` element.
Move the insertion of the `SyntacticElement` locator
element for the statement into the
SingleValueStmtExpr constraint generation function,
and remove it from the statement visitors themselves.
For the other cases, the `SyntacticElement` was
already being inserted for children of a BraceStmt,
so we were actually previously ending up with
duplicated elements in a couple of places.
