[InstCombine] be more careful when transforming a shuffle mask

This is reduced from a fuzzer test:
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=14890

Usually, demanded elements should be able to simplify shuffle
mask elements that are pointing to undef elements of its source
operands, but that doesn't happen in the test case.

llvm-svn: 361533