GlobalISel: remove "unsized" LLT

It was only really there as a sentinel when instructions had to have precisely
one type. Now that registers are typed, each register really has to have a type
that is sized.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@281599 91177308-0d34-0410-b5e6-96231b3b80d8
10 files changed