[PGO] Minor cleanup for count instruction in SelectInstVisitor.

[xur]

Summary

NSIs can be double-counted by different operations in
SelectInstVisitor. Sink the the update to VM_counting mode only.
Also reset the value for each counting operation.

[davidxl]

lgtm.

lib/Transforms/Instrumentation/PGOInstrumentation.cpp

struct SelectInstVisitor : public InstVisitor<SelectInstVisitor> {
   unsigned TotalNumCtrs = 0;     // Total number of counters
   GlobalVariable *FuncNameVar = nullptr;
   uint64_t FuncHash = 0;
   PGOUseFunc *UseFunc = nullptr;
 
   SelectInstVisitor(Function &Func) : F(Func) {}
 
   void countSelects(Function &Func) {
+    NSIs = 0;
     Mode = VM_counting;
     visit(Func);
   }
   // Visit the IR stream and instrument all select instructions. \p
   // Ind is a pointer to the counter index variable; \p TotalNC
   // is the total number of counters; \p FNV is the pointer to the
   // PGO function name var; \p FHash is the function hash.
   void instrumentSelects(Function &Func, unsigned *Ind, unsigned TotalNC,
void annotateSelects(Function &Func, PGOUseFunc *UF, unsigned *Ind) {
     CurCtrIdx = Ind;
     visit(Func);
   }
 
   void instrumentOneSelectInst(SelectInst &SI);
   void annotateOneSelectInst(SelectInst &SI);
   // Visit \p SI instruction and perform tasks according to visit mode.
   void visitSelectInst(SelectInst &SI);
+  // Return the number of select instructions. This needs be called after
+  // countSelects().
   unsigned getNumOfSelectInsts() const { return NSIs; }
 };
 
 class PGOInstrumentationGenLegacyPass : public ModulePass {
 public:
   static char ID;
 
   PGOInstrumentationGenLegacyPass() : ModulePass(ID) {

 
 void SelectInstVisitor::visitSelectInst(SelectInst &SI) {
   if (!PGOInstrSelect)
     return;
   // FIXME: do not handle this yet.
   if (SI.getCondition()->getType()->isVectorTy())
     return;
 
-  NSIs++;
   switch (Mode) {
   case VM_counting:
+    NSIs++;
     return;
   case VM_instrument:
     instrumentOneSelectInst(SI);
     return;
   case VM_annotate:
     annotateOneSelectInst(SI);
     return;
   }