[X86] Various improvements to our vector splitting helpers for lowering. NFC

-Consistently name the functions as split*
-Add a helper for doing the two extractSubvector calls and determining the size of the split
-Use getSplitDestVTs to get the result type for the split node.
-Move the binary and unary helper to one place in the file near the extractSubvector functions. Left the VSETCC one near LowerVSETCC since that's its only caller.
-Remove the 256/512 wrappers that just had asserts. I don't think they provided a lot of value and now with the routines called split* the call sites are more obvious what they do.
-Make the unary routine support different source and dest types to support D76212.
-Add some weaker asserts into the helpers to make up for losing the very specific asserts from the 256/512 wrappers.

Differential Revision: https://reviews.llvm.org/D78176