PPC: mark index input as unsigned in Store8Lane

Change-Id: Iefd76cdf793578acf46717d99c8896c7df7e1970
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3145179Reviewed-by: Junliang Yan <junyan@redhat.com>
Commit-Queue: Milad Fa <mfarazma@redhat.com>
Cr-Commit-Position: refs/heads/main@{#76705}

src/compiler/backend/ppc/code-generator-ppc.cc

@@ -3481,7 +3481,7 @@ CodeGenerator::CodeGenResult CodeGenerator::AssembleArchInstruction(
       MemOperand operand = i.MemoryOperand(&mode, &index);
       DCHECK_EQ(mode, kMode_MRR);
       __ vextractub(kScratchSimd128Reg, i.InputSimd128Register(0),
-                    Operand(15 - i.InputInt8(3)));
+                    Operand(15 - i.InputUint8(3)));
       __ stxsibx(kScratchSimd128Reg, operand);
       break;
     }