Port over the following:
- shuffle undef, undef, any_mask -> undef
- shuffle anything, anything, undef_mask -> undef
This sort of thing shows up a lot when you try to bugpoint code containing shufflevector. This can be annoying when you are doing things like "give me a testcase where GISel produces more instructions".
Also add a FIXME noting that these should be ported over to the custom predicate feature once it lands.