[Types] Define a getWithNewBitWidth for Types and make use of it

This is designed to change the bitwidth of a type without altering the number
of vector lanes. Also useful in D68651. Otherwise an NFC.

Differential Revision: https://reviews.llvm.org/D69139


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@375417 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed