GlobalISel: allow CodeGen to fallback on VReg type/class issues.

GlobalISel: allow CodeGen to fallback on VReg type/class issues.

After instruction selection we perform some checks on each VReg just before
discarding the type information. These checks were assertions before, but that
breaks the fallback path so this patch moves the logic into the main flow and
reports a better error on failure.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@286289 91177308-0d34-0410-b5e6-96231b3b80d8