[InstCombine] guard against splat-mul corner case

The test is already simplified, and I'm not sure how
to write a test to exercise the new clause. But it
protects the 2-bit pattern from miscompiling as noted
in D123453.

https://alive2.llvm.org/ce/z/QPyVfv
(If we managed to fall into the mul transform, it
would wrongly create a zero on this pattern.)