Reland r301880(!): "[InstSimplify] Handle selects of GEPs with 0 offset"

I was reminded today that this patch got reverted in r301885. I can no
longer reproduce the failure that caused the revert locally (...almost
one year later), and the patch applied pretty cleanly, so I guess we'll
see if the bots still get angry about it.

The original breakage was InstSimplify complaining (in "assertion
failed" form) about getting passed some crazy IR when running `ninja
check-sanitizer`. I'm unable to find traces of what, exactly, said crazy
IR was. I suppose we'll find out pretty soon if that's still the case.
:)

Original commit:

  Author: gbiv
  Date: Mon May  1 18:12:08 2017
  New Revision: 301880

  URL: http://llvm.org/viewvc/llvm-project?rev=301880&view=rev
  Log:
  [InstSimplify] Handle selects of GEPs with 0 offset

  In particular (since it wouldn't fit nicely in the summary):
  (select (icmp eq V 0) P (getelementptr P V)) -> (getelementptr P V)

  Differential Revision: https://reviews.llvm.org/D31435

llvm-svn: 330667

1 files changed, 13 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/InstructionSimplify.cpp b/llvm/lib/Analysis/InstructionSimplify.cpp
index 1e62b9d407e..25c38c06223 100644
--- a/llvm/lib/Analysis/InstructionSimplify.cpp
+++ b/llvm/lib/Analysis/InstructionSimplify.cpp
@@ -62,6 +62,8 @@ static Value *SimplifyOrInst(Value *, Value *, const SimplifyQuery &, unsigned);
 static Value *SimplifyXorInst(Value *, Value *, const SimplifyQuery &, unsigned);
 static Value *SimplifyCastInst(unsigned, Value *, Type *,
                                const SimplifyQuery &, unsigned);
+static Value *SimplifyGEPInst(Type *, ArrayRef<Value *>, const SimplifyQuery &,
+                              unsigned);
 
 /// For a boolean type or a vector of boolean type, return false or a vector
 /// with every element false.
@@ -3528,6 +3530,17 @@ static const Value *SimplifyWithOpReplaced(Value *V, Value *Op, Value *RepOp,
     }
   }
 
+  // Same for GEPs.
+  if (auto *GEP = dyn_cast<GetElementPtrInst>(I)) {
+    if (MaxRecurse) {
+      SmallVector<Value *, 8> NewOps(GEP->getNumOperands());
+      transform(GEP->operands(), NewOps.begin(),
+                [&](Value *V) { return V == Op ? RepOp : V; });
+      return SimplifyGEPInst(GEP->getSourceElementType(), NewOps, Q,
+                             MaxRecurse - 1);
+    }
+  }
+
   // TODO: We could hand off more cases to instsimplify here.
 
   // If all operands are constant after substituting Op for RepOp then we can