[InstCombine] Don't use getFirstNonPHI in FoldIntegerTypedPHI

getFirstNonPHI iterates over all the instructions in a block until it
finds a non-PHI.

Then, the loop starts from the beginning of the block and goes through
all the instructions until it reaches the instruction found by
getFirstNonPHI.

Instead of doing that, just stop when a non-PHI is found.

This reduces the compile-time of a test case discussed in
https://reviews.llvm.org/D47023 by 13x.

Not entirely sure how to come up with a test case for this since it's a
compile time issue that would significantly slow down running the tests.

Differential Revision: https://reviews.llvm.org/D70016

(cherry picked from commit a4c76be)

rdar://47501426

Author: francisvm

llvm/lib/Transforms/InstCombine/InstCombinePHI.cpp

@@ -180,13 +180,14 @@ Instruction *InstCombiner::FoldIntegerTypedPHI(PHINode &PN) {
          "Not enough available ptr typed incoming values");
   PHINode *MatchingPtrPHI = nullptr;
   unsigned NumPhis = 0;
-  for (auto II = BB->begin(), EI = BasicBlock::iterator(BB->getFirstNonPHI());
-       II != EI; II++, NumPhis++) {
+  for (auto II = BB->begin(); II != BB->end(); II++, NumPhis++) {
     // FIXME: consider handling this in AggressiveInstCombine
+    PHINode *PtrPHI = dyn_cast<PHINode>(II);
+    if (!PtrPHI)
+      break;
     if (NumPhis > MaxNumPhis)
       return nullptr;
-    PHINode *PtrPHI = dyn_cast<PHINode>(II);
-    if (!PtrPHI || PtrPHI == &PN || PtrPHI->getType() != IntToPtr->getType())
+    if (PtrPHI == &PN || PtrPHI->getType() != IntToPtr->getType())
       continue;
     MatchingPtrPHI = PtrPHI;
     for (unsigned i = 0; i != PtrPHI->getNumIncomingValues(); ++i) {