[llvm] [AArch64][SVE] Fold nested boolean UMIN trees (PR #209234)

[Paul Walker via llvm-commits]

================
@@ -3235,6 +3235,48 @@ instCombineInStreamingMode(InstCombiner &IC, IntrinsicInst &II) {
   return std::nullopt;
 }
 
+static std::optional<Instruction *> instCombineSVEUMin(InstCombiner &IC,
+                                                       IntrinsicInst &II) {
+  // Fold umin(umin(umin(umin(A, B), 1),
+  //                umin(umin(C, D), 1)),
+  //           1)
+  // into
+  //      umin(umin(umin(A, B),
+  //                umin(C, D)),
+  //           1),
----------------
paulwalker-arm wrote:

Rather than matching the whole pattern, if you split it into smaller combines it will simplify the code and has the chance to trigger in more cases.  I'm thinking something like:
```
umin(umin(A, 1), umin(B, 1)) -> umin(umin(A,B), 1)
umin(umin(A, 1), 1) -> umin(A, 1)
```

https://github.com/llvm/llvm-project/pull/209234