[IR] Extract helper for GEPNoWrapFlags intersection (NFC)

When combining two geps into one by adding the offsets, we have
to take some care when intersecting the flags, because nusw flags
cannot be straightforwardly preserved.

Add a helper for this on GEPNoWrapFlags so we won't have to repeat
this logic in various places.

Author: nikic

llvm/include/llvm/IR/GEPNoWrapFlags.h

@@ -74,6 +74,16 @@ class GEPNoWrapFlags {
     return GEPNoWrapFlags(Flags & ~NUWFlag);
   }
 
+  /// Given (gep (gep p, x), y), determine the nowrap flags for (gep p, x+y).
+  GEPNoWrapFlags intersectForOffsetAdd(GEPNoWrapFlags Other) const {
+    GEPNoWrapFlags Res = *this & Other;
+    // Without inbounds, we could only preserve nusw if we know that x + y does
+    // not wrap.
+    if (!Res.isInBounds() && Res.hasNoUnsignedSignedWrap())
+      Res = Res.withoutNoUnsignedSignedWrap();
+    return Res;
+  }
+
   bool operator==(GEPNoWrapFlags Other) const { return Flags == Other.Flags; }
   bool operator!=(GEPNoWrapFlags Other) const { return !(*this == Other); }
 

llvm/lib/Transforms/InstCombine/InstructionCombining.cpp

@@ -2349,12 +2349,7 @@ Instruction *InstCombinerImpl::narrowMathIfNoOverflow(BinaryOperator &BO) {
 /// transform.
 static GEPNoWrapFlags getMergedGEPNoWrapFlags(GEPOperator &GEP1,
                                               GEPOperator &GEP2) {
-  GEPNoWrapFlags NW = GEP1.getNoWrapFlags() & GEP2.getNoWrapFlags();
-  // Without inbounds, we could only preserve nusw if we know that x + y does
-  // not wrap.
-  if (!NW.isInBounds())
-    NW = NW.withoutNoUnsignedSignedWrap();
-  return NW;
+  return GEP1.getNoWrapFlags().intersectForOffsetAdd(GEP2.getNoWrapFlags());
 }
 
 /// Thread a GEP operation with constant indices through the constant true/false