IR: Allow vectors of halfs to be ConstantDataVectors

Currently, vectors of halfs end up as ConstantVectors, but there isn't
a good reason they can't be ConstantDataVectors. This should save some
memory.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@254991 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed