As noted in rL333782, we can be both better for optimization and safer with this transform:
BinOp (shuffle V1, Mask), C --> shuffle (BinOp V1, NewC), Mask
The only potentially unsafe-to-speculate binops are integer div/rem. All other binops are always safe (although I don't see a way to assert that in code here).
For opcodes like shifts that can produce poison, it can't matter here because we know the lanes with undef are dropped by the subsequent shuffle.