Fix DBC double comparisons for NaN.

Because NaN is not handled in a separate branch (like on other architetures -
e.g. branch-on-parity-flag on ia32), and every comparison with NaN returns false,
except "!=" we can't flip the branch condition in the same way for double
compares like we do for integer compares.

We could re-order blocks to always make the false-successor the fall-through, but
for now I just disable flipping the branch condition for double compares.

BUG=#27481
R=zra@google.com

Review URL: https://codereview.chromium.org/2400103002 .

runtime/vm/intermediate_language_dbc.cc

@@ -454,7 +454,7 @@ static void EmitBranchOnCondition(FlowGraphCompiler* compiler,
     }
   } else {
     ASSERT(true_condition == NEXT_IS_FALSE);
-    // NEXT_IS_FALSE indicates that the preceeing test has been flipped and
+    // NEXT_IS_FALSE indicates that the preceeding test has been flipped and
     // expects the false case to be in the subsequent instruction, which it
     // skips if the test succeeds.
     __ Jump(labels.false_label);
@@ -479,7 +479,7 @@ Condition StrictCompareInstr::EmitComparisonCode(FlowGraphCompiler* compiler,
     condition = NEXT_IS_TRUE;
     comparison = kind();
   } else {
-    // Flip comparision to save a jump.
+    // Flip comparison to save a jump.
     condition = NEXT_IS_FALSE;
     comparison = (kind() == Token::kEQ_STRICT) ? Token::kNE_STRICT
                                                : Token::kEQ_STRICT;
@@ -1798,15 +1798,12 @@ static Condition EmitDoubleComparisonOp(FlowGraphCompiler* compiler,
   const Register left = locs->in(0).reg();
   const Register right = locs->in(1).reg();
   Token::Kind comparison = kind;
+  // For double comparisons we can't flip the condition like with smi
+  // comparisons because of NaN which will compare false for all except !=
+  // operations.
+  // TODO(fschneider): Change the block order instead in DBC so that the
+  // false block in always the fall-through block.
   Condition condition = NEXT_IS_TRUE;
-  if (labels.fall_through != labels.false_label) {
-    // If we aren't falling through to the false label, we can save a Jump
-    // instruction in the case that the true case is the fall through by
-    // flipping the sense of the test such that the instruction following the
-    // test is the Jump to the false label.
-    condition = NEXT_IS_FALSE;
-    comparison = FlipCondition(kind);
-  }
   __ Emit(Bytecode::Encode(OpcodeForDoubleCondition(comparison), left, right));
   return condition;
 }

tests/lib/lib.status

@@ -396,9 +396,6 @@ convert/utf85_test: Pass, Timeout
 # version.
 mirrors/accessor_cache_overflow_test: Skip
 
-[ $arch == simdbc64 ]
-math/double_pow_test: Fail # Issue #27481
-
 [ $hot_reload || $hot_reload_rollback ]
 async/multiple_timer_test: Pass, Fail # Timing related
 async/stream_transformer_test: Pass, Fail # Closure identity