[llvm] [InstCombine] Do not request non-splat vector support in code reviews (NFC) (PR #90709)

[via llvm-commits]

================
@@ -554,3 +554,11 @@ guidelines.
    use of ValueTracking queries. Whether this makes sense depends on the case,
    but it's usually a good idea to only handle the constant pattern first, and
    then generalize later if it seems useful.
+
+## Guidelines for reviewers
+
+ * Do not ask contributors to implement non-splat vector support in code
----------------
goldsteinn wrote:

maybe this should be "new reviewers". IIUC the motivation is that its complicated and may be too much for a novice.

https://github.com/llvm/llvm-project/pull/90709