Don't build switch tables for dllimport and TLS variables in GEPs

This is a follow-up to r211331, which failed to notice that we were
returning early from ValidLookupTableConstant for GEPs.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211753 91177308-0d34-0410-b5e6-96231b3b80d8