Revert r162101 and replace it with a solution that works for targets where the pointe...

Revert r162101 and replace it with a solution that works for targets where the pointer type is illegal.
This is a horrible bit of code.  We're calling a simplification routine *in the middle* of type legalization.  We tell the
simplification routine that it's running after legalization, but some of the types it will encounter will be illegal!  The
fix is only to invoke the simplification if the types in question were legal, so that none of its invariants will be violated.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199847 91177308-0d34-0410-b5e6-96231b3b80d8