ScopInfo: Remove domains of error blocks (and blocks they dominate) early on

Trying to build up access functions for any of these blocks is likely to fail,
as error blocks may contain invalid/non-representable instructions, and blocks
dominated by error blocks may reference such instructions, which wil also cause
failures. As all of these blocks are anyhow assumed to not be executed, we can
just remove them early on.

This fixes http://llvm.org/PR25596

llvm-svn: 253818