[llvm] [VectorCombine] Fold chain of (scalar load)->ext->ext to load->ext. (PR #141109)

[Simon Pilgrim via llvm-commits]

================
@@ -2777,6 +2778,57 @@ bool VectorCombine::foldShuffleToIdentity(Instruction &I) {
   return true;
 }
 
+bool VectorCombine::foldShuffleExtExtracts(Instruction &I) {
+  // Try to fold vector zero- and sign-extends split across multiple operations
+  // into a single extend, removing redundant inserts and shuffles.
+
+  // Check if we have an extended shuffle that selects the first vector, which
+  // itself is another extend fed by a load.
+  Instruction *L;
+  if (!match(
+          &I,
+          m_OneUse(m_Shuffle(
+              m_OneUse(m_ZExtOrSExt(m_OneUse(m_BitCast(m_OneUse(m_InsertElt(
+                  m_Value(), m_OneUse(m_Instruction(L)), m_SpecificInt(0))))))),
----------------
RKSimon wrote:

(side note) it's a pity we can't do this:
```
LoadInst *L;
m_Load(L)
```

https://github.com/llvm/llvm-project/pull/141109