[X86] Lane-permute shuffle lowering can cycle indefinitely during legalization

X86 lane-permute shuffle lowering can cause legalization to cycle indefinitely for some vector shuffle masks.

When a generated cross-lane shuffle reproduces the original mask, trying a finer sublane size can produce a decomposition that legalization transforms back into the original shuffle. As a result, `llc` hangs instead of completing code generation.

https://godbolt.org/z/GxW3xKzKb