[llvm] fix a bug of PPCMIPeepholes which description in issue 71030 (PR #85451)

[Amy Kwan via llvm-commits]

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@@ -525,6 +525,18 @@ def getAltVSXFMAOpcode : InstrMapping {
   let ValueCols = [["1"]];
 }
 
+def get64BitInstrFromSignedExt32BitInstr : InstrMapping {
+  let FilterClass = "SExt32To64";
+  // Instructions with the same opcode.
+  let RowFields = ["Inst"];
+  // Instructions with the same Interpretation64Bit value form a column.
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amy-kwan wrote:

What does "form a column" mean?

https://github.com/llvm/llvm-project/pull/85451