[InstCombine] avoid infinite loop from vector select transforms

The 2nd test is based on the fuzzer example in post-commit
comments of D101191 -
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=34661

The 1st test shows that we don't deal with this symmetrically.
We should be able to reduce both examples (possibly in
instsimplify instead of instcombine).