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: efriedma, sdesmalen, rriddle

Reviewed By: sdesmalen

Subscribers: hiraditya, dantrushin, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D77261