[llvm] [LV] Improve AnyOf reduction codegen. (PR #78304)

[Arthur Eubanks via llvm-commits]

aeubanks wrote:

toward the end of the IR, we branch based on `all_same_length`. in the good IR with this patch reverted, I believe that's

`%bc.merge.rdx22 = phi i1 [ true, %iter.check ], [ %rdx.select, %vec.epilog.iter.check ], [ %rdx.select20, %vec.epilog.middle.block ]`

and in the bad IR I believe it's 

`%bc.merge.rdx21 = phi i1 [ true, %iter.check ], [ true, %vec.epilog.iter.check ], [ %rdx.select19, %vec.epilog.middle.block ]`

the `[ true, %vec.epilog.iter.check ]` is suspicious

https://github.com/llvm/llvm-project/pull/78304