[GlobalOpt] Fix exponential compile-time with selects.

If you have a long chain of select instructions created from something
like `int* p = &g; if (foo()) p += 4; if (foo2()) p += 4;` etc., a naive
recursive visitor will recursively visit each select twice, which is
O(2^N) in the number of select instructions. Use the visited set to cut
off recursion in this case.

(No testcase because this doesn't actually change the behavior, just the
time.)

Differential Revision: https://reviews.llvm.org/D42451

llvm-svn: 323910