[InstCombine] Fix crash due to early extractvalue removal

Fixes the issue reported at https://github.com/llvm/llvm-project/commit/4b8320868c9e32d1448c81ca76dba2a6b9f374cd#commitcomment-114671248.

The extractvalue instructions may still be used by the calling code
in some cases. Rather than trying to figure out which extracts are
safe to remove and which aren't, add them to the worklist so they
will get DCEd by the main loop.