ARM ISel: Don't create illegal types during LowerMUL

The transformation happening here is that we want to turn a
"mul(ext(X), ext(X))" into a "vmull(X, X)", stripping off the extension. We have
to make sure that X still has a valid vector type - possibly recreate an
extension to a smaller type. In case of a extload of a memory type smaller than
64 bit we used create a ext(load()). The problem with doing this - instead of
recreating an extload - is that an illegal type is exposed.

This patch fixes this by creating extloads instead of ext(load()) sequences.

Fixes PR15970.

radar://13871383

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181842 91177308-0d34-0410-b5e6-96231b3b80d8