Clean up usages of asserting vector getters in Type

Summary:
Remove usages of asserting vector getters in Type in preparation for the
VectorType refactor. The existence of these functions complicates the
refactor while adding little value.

Reviewers: sdesmalen, efriedma, krememek

Reviewed By: sdesmalen, efriedma

Subscribers: dexonsmith, Charusso, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D77257