Fix some places where we were assuming that memory type had been legalized

Fix some places where we were assuming that memory type had been legalized
to a simple type when lowering a truncating store of a vector type. In this
case for an EVT we'll return Expand as we should in all of the cases anyhow.

The testcase triggered at the one in VectorLegalizer::LegalizeOp, inspection
found the rest.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@254061 91177308-0d34-0410-b5e6-96231b3b80d8