Fix implicit conversion loses integer precision warning

Bug: chromium:1124085
Change-Id: I92bea75caad4ffc99d3890440295115f6d65e849
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2393216Reviewed-by: Adam Klein <adamk@chromium.org>
Reviewed-by: Bill Budge <bbudge@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Adam Klein <adamk@chromium.org>
Auto-Submit: Zequan Wu <zequanwu@google.com>
Cr-Commit-Position: refs/heads/master@{#69708}

src/compiler/backend/arm64/code-generator-arm64.cc

@@ -2562,7 +2562,7 @@ CodeGenerator::CodeGenResult CodeGenerator::AssembleArchInstruction(
                       src1 = i.InputSimd128Register(1).V16B();
       // Unary shuffle table is in src0, binary shuffle table is in src0, src1,
       // which must be consecutive.
-      int64_t mask = 0;
+      uint32_t mask = 0;
       if (src0 == src1) {
         mask = 0x0F0F0F0F;
       } else {