[turbofan] Only create Phis for live registers

In graph building, use liveness information to only create Phis
for live registers when merging two environments, or when preparing
a loop header. Similarly, only create loop exit value nodes for
live registers when exiting a loop.

Bug: v8:6701
Change-Id: Id6ac6a5518e6f1762530d871e92a46f42397928b
Reviewed-on: https://chromium-review.googlesource.com/615173
Commit-Queue: Jaroslav Sevcik <jarin@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47389}

src/compiler/bytecode-graph-builder.h

@@ -242,8 +242,9 @@ class BytecodeGraphBuilder {
   // Builds loop exit nodes for every exited loop between the current bytecode
   // offset and {target_offset}.
   void BuildLoopExitsForBranch(int target_offset);
-  void BuildLoopExitsForFunctionExit();
-  void BuildLoopExitsUntilLoop(int loop_offset);
+  void BuildLoopExitsForFunctionExit(const BytecodeLivenessState* liveness);
+  void BuildLoopExitsUntilLoop(int loop_offset,
+                               const BytecodeLivenessState* liveness);
 
   // Simulates entry and exit of exception handlers.
   void ExitThenEnterExceptionHandlers(int current_offset);