ARM64: Sign extension on MemOperand for keyed ops

src/arm64/assembler-arm64.h

Index: src/arm64/assembler-arm64.h
diff --git a/src/arm64/assembler-arm64.h b/src/arm64/assembler-arm64.h
index d2b1bca97f147477cc8e29284ee799bb21e3e350..53c805d8f8270bf50e6202176221fdc122495060 100644
--- a/src/arm64/assembler-arm64.h
+++ b/src/arm64/assembler-arm64.h
@@ -669,6 +669,7 @@ class Operand {
 // MemOperand represents a memory operand in a load or store instruction.
 class MemOperand {
  public:
+  inline explicit MemOperand();
   inline explicit MemOperand(Register base,
                              ptrdiff_t offset = 0,
                              AddrMode addrmode = Offset);