Subzero: Convert NULL->nullptr.

This is limited to the src directory, and not (yet) the crosstest directory.

BUG= none
R=jfb@chromium.org

Review URL: https://codereview.chromium.org/814353002
diff --git a/src/IceTargetLowering.cpp b/src/IceTargetLowering.cpp
index be1d2e1..c20f9f6 100644
--- a/src/IceTargetLowering.cpp
+++ b/src/IceTargetLowering.cpp
@@ -98,7 +98,7 @@
     return IceTargetARM64::create(Func);
 #endif
   Func->setError("Unsupported target");
-  return NULL;
+  return nullptr;
 }
 
 TargetLowering::TargetLowering(Cfg *Func)
@@ -113,7 +113,7 @@
   if (Target == Target_X8632)
     return new x86::AssemblerX86();
   Func->setError("Unsupported target");
-  return NULL;
+  return nullptr;
 }
 
 void TargetLowering::doAddressOpt() {
@@ -267,7 +267,7 @@
     return IceTargetGlobalInitARM64::create(Ctx);
 #endif
   llvm_unreachable("Unsupported target");
-  return NULL;
+  return nullptr;
 }
 
 TargetGlobalInitLowering::~TargetGlobalInitLowering() {}