[NFC][CLANG] Fix nullptr dereference issue in checkSizelessVectorShift()

This patch uses castAs instead of getAs which will assert if the type doesn't match in checkSizelessVectorShift(clang::Sema &, clang::ActionResult<clang::Expr *, true> &, clang::ActionResult<clang::Expr *, true> &, clang::SourceLocation, bool).

Reviewed By: erichkeane

Differential Revision: https://reviews.llvm.org/D152107