[Polly] Remove checkIslAstExprInt and use RAII instead of manually freeing Expr. NFC.

Polly is trying to move towards using isl::ast_expr / isl-noexceptions.h
(which implements RAII) where possible instead of manually managing memory.
checkIslAstExprInt manually frees Expr, so it has been removed to be
more idiomatic and consistent.

Reviewed By: Meinersbur

Differential Revision: https://reviews.llvm.org/D111769