[Bug target/110751] RISC-V: Suport undefined value that allows VSETVL PASS use TA/MA

[rguenth at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110751

--- Comment #31 from Richard Biener <rguenth at gcc dot gnu.org> ---
On GIMPLE an "undefined" operand representation would be the default definition
of an SSA name with the appropriate type.  That's a somewhat "heavy"
representation and it also doesn't fit the target hook return value nicely,
but we could handle a NULL_TREE return value from the target hook in the
way to create such SSA name.