[arm] Fix a double-register push operation.

src/arm/assembler-arm.cc

Index: src/arm/assembler-arm.cc
diff --git a/src/arm/assembler-arm.cc b/src/arm/assembler-arm.cc
index b5910d05be5de63c9092aedecdec05ec9e487239..50c707d2a0741c88c1b9efc3a8db6183be147fed 100644
--- a/src/arm/assembler-arm.cc
+++ b/src/arm/assembler-arm.cc
@@ -302,6 +302,13 @@ MemOperand::MemOperand(Register rn, int32_t offset, AddrMode am) {
   rm_ = no_reg;
   offset_ = offset;
   am_ = am;
+
+  // Accesses below the stack pointer are not safe, and are prohibited by the
+  // ABI. We can check obvious violations here.
+  if (rn.is(sp)) {
+    if (am == Offset) DCHECK_LE(0, offset);
+    if (am == NegOffset) DCHECK_GE(0, offset);
+  }
 }