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: arsenm, efriedma, sdesmalen

Reviewed By: arsenm

Subscribers: wdng, arsenm, jvesely, nhaehnle, hiraditya, kerbowa, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D77268