[llvm] [SLP]Model single unique value insert + shuffle as splat + select, where profitable (PR #136590)

[Alexey Bataev via llvm-commits]

alexey-bataev wrote:

> It looks like all of your test changes are analogous to a vmerge.vxm on RISCV - that is, an identity shuffle with a conditionally masked broadcast overwriting some lanes. This change seems to handle all shuffles with broadcasts, would it maybe make sense to do something more restrictive? Not sure what your motivating case is, but the broader the change, the more likely we'll have some unexpected effect.

This change is pretty straightforward, I can limit only only by dropping the broadcast

https://github.com/llvm/llvm-project/pull/136590