RFR [8u] Restore accidental revert of JDK-8180855

[Aleksey Shipilev]

Accidentally reverted this during the merge:
  http://hg.openjdk.java.net/jdk9/jdk9/hotspot/rev/1ae9e84f68b3

Restoring:

diff -r 8d009c095cb8 src/share/vm/compiler/oopMap.cpp
--- a/src/share/vm/compiler/oopMap.cpp	Wed Mar 21 19:30:30 2018 +0100
+++ b/src/share/vm/compiler/oopMap.cpp	Wed Mar 21 19:47:22 2018 +0100
@@ -389,17 +389,16 @@
         omv = oms.current();
         oop* loc = fr->oopmapreg_to_location(omv.reg(),reg_map);
         if ( loc != NULL ) {
+          oop *derived_loc = loc;
           oop *base_loc    = fr->oopmapreg_to_location(omv.content_reg(), reg_map);
-          oop *derived_loc = loc;
-          oop val = *base_loc;
-          if (val == NULL || Universe::is_narrow_oop_base(val)) {
-            // Ignore NULL oops and decoded NULL narrow oops which
-            // equal to Universe::narrow_oop_base when a narrow oop
-            // implicit null check is used in compiled code.
-            // The narrow_oop_base could be NULL or be the address
-            // of the page below heap depending on compressed oops mode.
-          } else
+          // Ignore NULL oops and decoded NULL narrow oops which
+          // equal to Universe::narrow_oop_base when a narrow oop
+          // implicit null check is used in compiled code.
+          // The narrow_oop_base could be NULL or be the address
+          // of the page below heap depending on compressed oops mode.
+          if (base_loc != NULL && *base_loc != (oop)NULL && !Universe::is_narrow_oop_base(*base_loc)) {
             derived_oop_fn(base_loc, derived_loc);
+          }
         }
         oms.next();
       }  while (!oms.is_done());

Thanks,
-Aleksey