[builtins] Introduce proper Float64Log1p operator.

src/assembler.cc

Index: src/assembler.cc
diff --git a/src/assembler.cc b/src/assembler.cc
index ce755f2631ca6dc69d75185b7a55459f1e72bcb4..8fe196cabb1b488e03cac430231879e5e447d12e 100644
--- a/src/assembler.cc
+++ b/src/assembler.cc
@@ -1653,6 +1653,10 @@ ExternalReference ExternalReference::ieee754_log_function(Isolate* isolate) {
       Redirect(isolate, FUNCTION_ADDR(base::ieee754::log), BUILTIN_FP_CALL));
 }
 
+ExternalReference ExternalReference::ieee754_log1p_function(Isolate* isolate) {
+  return ExternalReference(
+      Redirect(isolate, FUNCTION_ADDR(base::ieee754::log1p), BUILTIN_FP_CALL));
+}
 
 ExternalReference ExternalReference::math_exp_constants(int constant_index) {
   DCHECK(math_exp_data_initialized);