An i64 AssertZExt from a type smaller than i32 has at least 33
leading zeros which mean it has at least 33 sign bits.
Since we have a couple patterns that use two sexti32, I've
switched to a ComplexPattern so tablegen didn't have to generate
9 different permutations.
As noted in the FIXME, maybe we should just call computeNumSignBits,
but we don't have tests that benefit from that yet.