Intrinisc type signature is a `list<list<int>>` that hold IIT encoding
for each param/ret type (outer list) where the IIT encoding for each
type itself can be 0 or more integers (the inner list). Intrinsic
emitter flatten this list into generate the type signature in
`ComputeTypeSignature`.
Use the new !listflatten() operator to instead flatten the list in the
TableGen definition and eliminate flattening in the emitter code.
Verified that `-gen-intrinsic-impl` output for Intrinsics.td is
identical with and without the change.
1c984b86b3