[SimplifyCFG] Add early bailout if Use is not in same BB.

Without this patch, passingValueIsAlwaysUndefined will iterate over all
instructions from I to the end of the basic block, even if the use is
outside the block.

This patch adds an early bail out, if the use instruction is outside I's
BB. This can greatly reduce compile-time in cases where very large basic
blocks are involved, with a large number of PHI nodes and incoming
values.

Note that the refactoring makes the handling of the case where I is a
phi and Use is in PHI more explicit  as well: for phi nodes, we can also
directly bail out. In the existing code, we would iterate until we reach
the end and return false.

Based on an earlier patch by Matt Wala.

Reviewed By: lebedev.ri

Differential Revision: https://reviews.llvm.org/D113293

Author: fhahn

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

@@ -6527,19 +6527,21 @@ static bool passingValueIsAlwaysUndefined(Value *V, Instruction *I, bool PtrValu
 
   if (C->isNullValue() || isa<UndefValue>(C)) {
     // Only look at the first use, avoid hurting compile time with long uselists
-    User *Use = *I->user_begin();
+    auto *Use = cast<Instruction>(*I->user_begin());
+    // Bail out if Use is not in the same BB as I or Use == I or Use comes
+    // before I in the block. The latter two can be the case if Use is a PHI
+    // node.
+    if (Use->getParent() != I->getParent() || Use == I || Use->comesBefore(I))
+      return false;
 
     // Now make sure that there are no instructions in between that can alter
     // control flow (eg. calls)
-    for (BasicBlock::iterator
-             i = ++BasicBlock::iterator(I),
-             UI = BasicBlock::iterator(dyn_cast<Instruction>(Use));
-         i != UI; ++i) {
-      if (i == I->getParent()->end())
-        return false;
-      if (!isGuaranteedToTransferExecutionToSuccessor(&*i))
-        return false;
-    }
+    auto InstrRange =
+        make_range(std::next(I->getIterator()), Use->getIterator());
+    if (any_of(InstrRange, [](Instruction &I) {
+          return !isGuaranteedToTransferExecutionToSuccessor(&I);
+        }))
+      return false;
 
     // Look through GEPs. A load from a GEP derived from NULL is still undefined
     if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(Use))