[mlir][ArmSME] Fix crash on func decls in 'arm_za' legality checks

[Dinistro]

LGTM!

mlir/lib/Dialect/ArmSME/Transforms/LegalizeForLLVMExport.cpp

void mlir::configureArmSMELegalizeForExportTarget(
   target.addLegalOp<arm_sme::aarch64_sme_za_enable,
                     arm_sme::aarch64_sme_za_disable>();
 
   // Mark 'func.func' ops as legal if either:
   //   1. no 'arm_za' function attribute is present.
   //   2. the 'arm_za' function attribute is present and the first op in the
   //      function is an 'arm_sme::aarch64_sme_za_enable' intrinsic.
   target.addDynamicallyLegalOp<func::FuncOp>([&](func::FuncOp funcOp) {
+    if (funcOp.isDeclaration())
+      return true;
     auto firstOp = funcOp.getBody().front().begin();
     return !funcOp->hasAttr("arm_za") ||
            isa<arm_sme::aarch64_sme_za_enable>(firstOp);
   });
 
   // Mark 'func.return' ops as legal if either:
   //   1. no 'arm_za' function attribute is present.
   //   2. the 'arm_za' function attribute is present and there's a preceding

mlir/test/Dialect/ArmSME/enable-arm-za.mlir

 // RUN: mlir-opt %s -enable-arm-streaming=enable-za -convert-vector-to-llvm="enable-arm-sme" | FileCheck %s -check-prefix=ENABLE-ZA
 // RUN: mlir-opt %s -enable-arm-streaming -convert-vector-to-llvm="enable-arm-sme" | FileCheck %s -check-prefix=DISABLE-ZA
 // RUN: mlir-opt %s -convert-vector-to-llvm="enable-arm-sme" | FileCheck %s -check-prefix=NO-ARM-STREAMING
 
+// CHECK-LABEL: @declaration
+func.func private @declaration()
+
 // CHECK-LABEL: @arm_za
 func.func @arm_za() {
   // ENABLE-ZA: arm_sme.intr.za.enable
   // ENABLE-ZA-NEXT: arm_sme.intr.za.disable
   // ENABLE-ZA-NEXT: return
   // DISABLE-ZA-NOT: arm_sme.intr.za.enable
   // DISABLE-ZA-NOT: arm_sme.intr.za.disable
   // NO-ARM-STREAMING-NOT: arm_sme.intr.za.enable
   // NO-ARM-STREAMING-NOT: arm_sme.intr.za.disable
   return
 }