[x86] test codegen for unsigned saturated add; NFC

All of the ISA holes are going to make this difficult,
but we can't canonicalize the IR and try to solve PR14613
until we have backend support to get this right.

https://bugs.llvm.org/show_bug.cgi?id=14613

https://rise4fun.com/Alive/Guv
https://rise4fun.com/Alive/AADG

llvm-svn: 341845