Type legalization will insert a sign extend anyway. By doing it
early we can remove the zext. ComputeNumSignBits can't spot it
after type legalization because type legalization may expand
the abs to sra+xor+sub.
If the zext result type is larger than the type to be promoted to,
we'll promote to a legal type and then zext the rest of the way.
If the legal type is larger than the destination type we can promote
and then truncate.