[MVT] Fix getTypeForEVT for v64f16 and v128f16

Summary: These should have half float as the element type

Reviewers: cameron.mcinally, efriedma, sdesmalen, paulwalker-arm

Reviewed By: paulwalker-arm

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D84211