[IR][NFC] Adds BasicBlock::erase().

[vporpo]

Summary

Currently the only way to do this is to work with the instruction list directly.
This is part of a series of cleanup patches towards making BasicBlock::getInstList() private.

llvm/include/llvm/IR/BasicBlock.h

public:
   }
 
   /// Transfer a range of instructions that belong to \p FromBB from \p
   /// FromBeginIt to \p FromEndIt, to this basic block at \p ToIt.
   void splice(BasicBlock::iterator ToIt, BasicBlock *FromBB,
               BasicBlock::iterator FromBeginIt,
               BasicBlock::iterator FromEndIt);
 
+  /// Erases a range of instructions from \p FromIt to (not including) \p ToIt.
+  /// \Returns \p ToIt.
+  BasicBlock::iterator erase(BasicBlock::iterator FromIt, BasicBlock::iterator ToIt);
+
   /// Returns true if there are any uses of this basic block other than
   /// direct branches, switches, etc. to it.
   bool hasAddressTaken() const {
     return getBasicBlockBits().BlockAddressRefCount != 0;
   }
 
   /// Update all phi nodes in this basic block to refer to basic block \p New
   /// instead of basic block \p Old.

llvm/lib/IR/BasicBlock.cpp

#ifdef EXPENSIVE_CHECKS
   // Check that FromBeginIt is befor FromEndIt.
   auto FromBBEnd = FromBB->end();
   for (auto It = FromBeginIt; It != FromEndIt; ++It)
     assert(It != FromBBEnd && "FromBeginIt not before FromEndIt!");
 #endif // EXPENSIVE_CHECKS
   getInstList().splice(ToIt, FromBB->getInstList(), FromBeginIt, FromEndIt);
 }
 
+BasicBlock::iterator BasicBlock::erase(BasicBlock::iterator FromIt,
+                                       BasicBlock::iterator ToIt) {
+  return getInstList().erase(FromIt, ToIt);
+}
+
 void BasicBlock::replacePhiUsesWith(BasicBlock *Old, BasicBlock *New) {
   // N.B. This might not be a complete BasicBlock, so don't assume
   // that it ends with a non-phi instruction.
   for (Instruction &I : *this) {
     PHINode *PN = dyn_cast<PHINode>(&I);
     if (!PN)
       break;
     PN->replaceIncomingBlockWith(Old, New);

llvm/unittests/IR/BasicBlockTest.cpp

)");
   Instruction *ToI2 = &*ToBBIt++;
 
   EXPECT_DEATH(ToBB->splice(ToI2->getIterator(), FromBB, FromI2->getIterator(),
                             FromI1->getIterator()),
                "FromBeginIt not before FromEndIt!");
 }
 #endif //EXPENSIVE_CHECKS
 
+TEST(BasicBlockTest, EraseRange) {
+  LLVMContext Ctx;
+  std::unique_ptr<Module> M = parseIR(Ctx, R"(
+    define void @f(i32 %a) {
+     bb0:
+       %instr1 = add i32 %a, %a
+       %instr2 = sub i32 %a, %a
+       ret void
+    }
+)");
+  Function *F = &*M->begin();
+
+  auto BB0It = F->begin();
+  BasicBlock *BB0 = &*BB0It;
+
+  auto It = BB0->begin();
+  Instruction *Instr1 = &*It++;
+  Instruction *Instr2 = &*It++;
+
+  EXPECT_EQ(BB0->size(), 3u);
+
+  // Erase no instruction
+  BB0->erase(Instr1->getIterator(), Instr1->getIterator());
+  EXPECT_EQ(BB0->size(), 3u);
+
+  // Erase %instr1
+  BB0->erase(Instr1->getIterator(), Instr2->getIterator());
+  EXPECT_EQ(BB0->size(), 2u);
+  EXPECT_EQ(&*BB0->begin(), Instr2);
+
+  // Erase all instructions
+  BB0->erase(BB0->begin(), BB0->end());
+  EXPECT_TRUE(BB0->empty());
+}
 } // End anonymous namespace.
 } // End llvm namespace.