[llvm] [GlobalISel][AArch64] Legalize G_EXTRACT_SUBVECTOR (PR #112946)

[Thorsten Schütt via llvm-commits]

================
@@ -1363,6 +1363,11 @@ AArch64LegalizerInfo::AArch64LegalizerInfo(const AArch64Subtarget &ST)
 
   getActionDefinitionsBuilder({G_SCMP, G_UCMP}).lower();
 
+  getActionDefinitionsBuilder(G_EXTRACT_SUBVECTOR)
+      .legalFor({{v8s8, v16s8}, {v4s16, v8s16}, {v2s32, v4s32}})
+      .widenScalarOrEltToNextPow2(0)
+      .immIdx(0); // Inform verifier imm idx 0 is handled.
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tschuett wrote:

```
 getLegacyLegalizerInfo().computeTables();
  verify(*ST.getInstrInfo());
```
Yes.

https://github.com/llvm/llvm-project/pull/112946