Clean up usages of asserting vector getters in Type
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
Differential Revision: https://reviews.llvm.org/D77268