[llvm] [Matrix] Propagate shape information through PHI insts (PR #141681)

[Jon Roelofs via llvm-commits]

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@@ -561,6 +590,27 @@ class LowerMatrixIntrinsics {
       MatrixVal = M.embedInVector(Builder);
     }
 
+    // If it's a PHI, split it now. We'll take care of fixing up the operands
+    // later once we're in VisitPHI.
+    if (auto *PHI = dyn_cast<PHINode>(MatrixVal)) {
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jroelofs wrote:

If the PHI is lowered after one of its users (which might be another PHI), then getMatrix would embed the matrix in a vector on behalf of the PHI which we want to replace, which is exactly what we're trying to avoid. I'm not sure it _needs_ to be here, but I don't see a better way to do it. Open to suggestions if you've got something in mind.

https://github.com/llvm/llvm-project/pull/141681