Fix large performance regression in Box2D, MD5, and SHA1 benchmarks

runtime/vm/intermediate_language_x64.cc

Index: runtime/vm/intermediate_language_x64.cc
diff --git a/runtime/vm/intermediate_language_x64.cc b/runtime/vm/intermediate_language_x64.cc
index 006c2d1ecab36c04dc2cebd61ebe9dfdb46b969a..5c670f819ef0647c959179c547444ac164984b22 100644
--- a/runtime/vm/intermediate_language_x64.cc
+++ b/runtime/vm/intermediate_language_x64.cc
@@ -1570,6 +1570,11 @@ void GuardFieldInstr::EmitNativeCode(FlowGraphCompiler* compiler) {
           // list length here, without this check the list length could change
           // without triggering a deoptimization.
           Label check_array, length_compared, no_fixed_length;
+          // If length is negative the length guard is either disabled or
+          // has not been initialized, either way it is safe to skip the
+          // length check.
+          __ cmpq(field_length_operand, Immediate(Smi::RawValue(0)));
+          __ j(LESS, &skip_length_check);
           __ cmpq(value_cid_reg, Immediate(kNullCid));
           __ j(EQUAL, &no_fixed_length, Assembler::kNearJump);
           // Check for typed data array.