[IR] Split vector.splice into vector.splice.left and vector.splice.right (#170796)

This PR implements the first change outlined in
https://discourse.llvm.org/t/rfc-allow-non-constant-offsets-in-llvm-vector-splice/88974?u=lukel

In order to allow non-immediate offsets in the llvm.vector.splice
intrinsic, we need to separate out the "shift left" and "shift right"
modes into two separate intrinsics, which were previously determined by
whether or not the offset is positive or negative.

The description in the LangRef has also been reworded in terms of
sliding elements left or right and extracting either the upper or lower
half as opposed to extracting from a certain index, which brings it
inline with the definition of `llvm.fshr.*`/`llvm.fshl.*`.

This patch teaches AutoUpgrade.cpp to upgrade the old intrinsics into
their new equivalent one based on their offset, so existing uses of
vector.splice should still work.

Uses of llvm.vector.splice in `llvm/test/CodeGen` haven't been replaced
in this PR to keep the diff small and kick the tyres on the AutoUpgrader
a bit. I planned to do this in a follow up NFC but can include it in
this PR if reviewers prefer.

Similarly the shuffle costing kind `SK_Splice` has just been kept the
same for now, to be split into `SK_SpliceLeft` and `SK_SpliceRight`
later.