PPC: Fix trampoline pool blocking.

Check whether a trampoline pool should be emitted after unblocking.
Otherwise, back-to-back sequences which block the trampoline pool can
cause it to be out of reach.

R=joransiu@ca.ibm.com, jyan@ca.ibm.com, michael_dawson@ca.ibm.com, dstence@us.ibm.com
BUG=

Review URL: https://codereview.chromium.org/1433343002

Cr-Commit-Position: refs/heads/master@{#31945}

src/ppc/assembler-ppc.h

@@ -1310,7 +1310,10 @@ class Assembler : public AssemblerBase {
   }
 
   void StartBlockTrampolinePool() { trampoline_pool_blocked_nesting_++; }
-  void EndBlockTrampolinePool() { trampoline_pool_blocked_nesting_--; }
+  void EndBlockTrampolinePool() {
+    int count = --trampoline_pool_blocked_nesting_;
+    if (count == 0) CheckTrampolinePoolQuick();
+  }
   bool is_trampoline_pool_blocked() const {
     return trampoline_pool_blocked_nesting_ > 0;
   }