Use new vector insert half-word and byte instructions when we see insertelement on...

Use new vector insert half-word and byte instructions when we see insertelement on '8 x i16' and '16 x i8' types.  Also extended existing lit testcase to cover these cases.

Differential Revision: https://reviews.llvm.org/D34630

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@317613 91177308-0d34-0410-b5e6-96231b3b80d8