[llvm] [X86] Combine bitcast(v1Ty insert_vector_elt(X, Y, 0)) to Y (PR #130475)

[Phoebe Wang via llvm-commits]

phoebewang wrote:

> Does this work as a generic DAGCombiner fold?

It does, but is not necessary for now, because only `v1i1` is a legal type to X86.

https://github.com/llvm/llvm-project/pull/130475