[clang] [CIR] Implement builtin extractf (PR #170427)

[Andy Kaylor via cfe-commits]

================
@@ -887,7 +921,30 @@ mlir::Value CIRGenFunction::emitX86BuiltinExpr(unsigned builtinID,
   case X86::BI__builtin_ia32_extractf64x2_256_mask:
   case X86::BI__builtin_ia32_extracti64x2_256_mask:
   case X86::BI__builtin_ia32_extractf64x2_512_mask:
-  case X86::BI__builtin_ia32_extracti64x2_512_mask:
+  case X86::BI__builtin_ia32_extracti64x2_512_mask: {
+    mlir::Location loc = getLoc(expr->getExprLoc());
+    cir::VectorType dstTy = cast<cir::VectorType>(convertType(expr->getType()));
+    unsigned numElts = dstTy.getSize();
+    unsigned srcNumElts = cast<cir::VectorType>(ops[0].getType()).getSize();
+    unsigned subVectors = srcNumElts / numElts;
+    assert(llvm::isPowerOf2_32(subVectors) && "Expected power of 2 subvectors");
+    unsigned index =
+        ops[1].getDefiningOp<cir::ConstantOp>().getIntValue().getZExtValue();
+
+    index &= subVectors - 1; // Remove any extra bits.
+    index *= numElts;
+
+    int64_t indices[16];
+    std::iota(indices, indices + numElts, index);
+
+    mlir::Value zero = builder.getNullValue(ops[0].getType(), loc);
----------------
andykaylor wrote:

```suggestion
    mlir::Value
    poison = builder.getConstant(loc, cir::PoisonAttr::get(ops[0].getType()));
```

https://github.com/llvm/llvm-project/pull/170427