[SeparateConstOffsetFromGEP] Fix an illegitimate optimization on zext

[jingyue]

Summary

zext(a + b) != zext(a) + zext(b) even if a + b >= 0 && b >= 0.

e.g., a = i4 0b1111, b = i4 0b0001
zext a + b to i8 = zext 0b0000 to i8 = 0b00000000
(zext a to i8) + (zext b to i8) = 0b00001111 + 0b00000001 = 0b00010000

[meheff]

LGTM

[jingyue]

Committed in r210439

lib/Transforms/Scalar/SeparateConstOffsetFromGEP.cpp

bool ConstantOffsetExtractor::CanTraceInto(bool SignExtended,
   // Suppose BO = A op B.
   //  SignExtended | ZeroExtended | Distributable?
   // --------------+--------------+----------------------------------
   //       0       |      0       | true because no s/zext exists
   //       0       |      1       | zext(BO) == zext(A) op zext(B)
   //       1       |      0       | sext(BO) == sext(A) op sext(B)
   //       1       |      1       | zext(sext(BO)) ==
   //               |              |     zext(sext(A)) op zext(sext(B))
-  if (BO->getOpcode() == Instruction::Add && NonNegative) {
+  if (BO->getOpcode() == Instruction::Add && !ZeroExtended && NonNegative) {
     // If a + b >= 0 and (a >= 0 or b >= 0), then
-    //   s/zext(a + b) = s/zext(a) + s/zext(b)
+    //   sext(a + b) = sext(a) + sext(b)
     // even if the addition is not marked nsw.
     //
     // Leveraging this invarient, we can trace into an sext'ed inbound GEP
     // index if the constant offset is non-negative.
     //
     // Verified in @sext_add in split-gep.ll.
     if (ConstantInt *ConstLHS = dyn_cast<ConstantInt>(LHS)) {
       if (!ConstLHS->isNegative())