Limit the number of blocks handled by TraceBuilder::optimizeTrace

It appears to be O(N^3) in the number of blocks (a quick glance
at the code suggested N^2, but 1000 blocks took about .5s to process
while 2000 blocks took 4; 19000 blocks was nowhere near finished in
an hour). A better representation of the dominators would probably
solve this - but would also take *much* more space.
Having more than 1000 blocks appears to be extremely rare, so
this seems like a reasonable solution for now.

hphp/runtime/base/runtime_option.h

@@ -414,6 +414,7 @@ public:
   F(bool, HHIRGenericDtorHelper,       true) \
   F(bool, HHIRCse,                     true) \
   F(bool, HHIRSimplification,          true) \
+  F(uint32_t, HHIRSimplificationMaxBlocks,1000) \
   F(bool, HHIRGenOpts,                 true) \
   F(bool, HHIRJumpOpts,                true) \
   F(bool, HHIRExtraOptPass,            true) \

hphp/runtime/vm/translator/hopt/tracebuilder.cpp

@@ -1522,6 +1522,12 @@ void TraceBuilder::optimizeTrace() {
   m_enableCse = RuntimeOption::EvalHHIRCse;
   m_enableSimplification = RuntimeOption::EvalHHIRSimplification;
   if (!m_enableCse && !m_enableSimplification) return;
+  if (m_trace->getBlocks().size() >
+      RuntimeOption::EvalHHIRSimplificationMaxBlocks) {
+    // TODO CSEHash::filter is very slow for large block sizes
+    // t2135219 should address that
+    return;
+  }
   BlockList sortedBlocks = sortCfg(m_trace.get(), m_irFactory);
   IdomVector idoms = findDominators(sortedBlocks);
   clearTrackedState();