Something like:
`%r = mul %x, <33, 33, 33, ...>`
Is best lowered as:
`%tmp = %shl x, <5, 5, 5>; %r = add %tmp, %x`
As well, since vectors have non-destructive shifts, we can also do
cases where the multiply constant is Pow2A +/- Pow2B for arbitrary A
and B, unlike in the scalar case where the extra mov instructions
make it not worth it.
Can we get such info from SchedModel rather than put another tuning?