[ConstantFolding] Set signed/implicitTrunc when handling GEP offsets

GEP offsets have sext_or_trunc semantics. We were already doing
this for the outer-most GEP, but not for the inner ones.

I believe one of the sanitizer buildbot failures was due to this,
but I did not manage to reproduce the issue or come up with a
test case. Usually the problematic case will already be folded
away due to index type canonicalization.

Author: nikic

llvm/lib/Analysis/ConstantFolding.cpp

@@ -924,7 +924,8 @@ Constant *SymbolicallyEvaluateGEP(const GEPOperator *GEP,
     Ptr = cast<Constant>(GEP->getOperand(0));
     SrcElemTy = GEP->getSourceElementType();
     Offset = Offset.sadd_ov(
-        APInt(BitWidth, DL.getIndexedOffsetInType(SrcElemTy, NestedOps)),
+        APInt(BitWidth, DL.getIndexedOffsetInType(SrcElemTy, NestedOps),
+              /*isSigned=*/true, /*implicitTrunc=*/true),
         Overflow);
   }