InstCombine: Don't assume DataLayout is always available

We tried to get the result of DataLayout::getLargestLegalIntTypeSize but
we didn't have a DataLayout.  This resulted in opt crashing.

This fixes PR21651.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222645 91177308-0d34-0410-b5e6-96231b3b80d8