Fix an instance of -Wunused-but-set-variable.

Bug: chromium:1203071
Change-Id: Iab81add0c71c4a37b7aa8d9d6f1272781a6bae3d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3152217
Auto-Submit: Peter Kasting <pkasting@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#76999}

src/diagnostics/arm/disasm-arm.cc

@@ -2026,13 +2026,11 @@ void Decoder::DecodeAdvancedSIMDDataProcessing(Instruction* instr) {
   int op1 = instr->Bit(4);
   if (op0 == 0) {
     // Advanced SIMD three registers of same length.
-    int Vd, Vm, Vn;
+    int Vm, Vn;
     if (instr->Bit(6) == 0) {
-      Vd = instr->VFPDRegValue(kDoublePrecision);
       Vm = instr->VFPMRegValue(kDoublePrecision);
       Vn = instr->VFPNRegValue(kDoublePrecision);
     } else {
-      Vd = instr->VFPDRegValue(kSimd128Precision);
       Vm = instr->VFPMRegValue(kSimd128Precision);
       Vn = instr->VFPNRegValue(kSimd128Precision);
     }