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: rriddle, efriedma, sdesmalen

Reviewed By: sdesmalen

Subscribers: frgossen, mehdi_amini, rriddle, jpienaar, burmako, shauheen, antiagainst, nicolasvasilache, arpith-jacob, mgester, lucyrfox, aartbik, liufengdb, Joonsoo, grosul1, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D77258