[CGP] Fix GEP issue with out of range APInt constant values not fitting in int64_t

Test case reduced from https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=7173

llvm-svn: 339556
diff --git a/llvm/lib/CodeGen/CodeGenPrepare.cpp b/llvm/lib/CodeGen/CodeGenPrepare.cpp
index 725b69d..2dbf263 100644
--- a/llvm/lib/CodeGen/CodeGenPrepare.cpp
+++ b/llvm/lib/CodeGen/CodeGenPrepare.cpp
@@ -3801,8 +3801,13 @@
       } else {
         uint64_t TypeSize = DL.getTypeAllocSize(GTI.getIndexedType());
         if (ConstantInt *CI = dyn_cast<ConstantInt>(AddrInst->getOperand(i))) {
-          ConstantOffset += CI->getSExtValue() * TypeSize;
-        } else if (TypeSize) {  // Scales of zero don't do anything.
+          const APInt &CVal = CI->getValue();
+          if (CVal.getMinSignedBits() <= 64) {
+            ConstantOffset += CVal.getSExtValue() * TypeSize;
+            continue;
+          }
+        }
+        if (TypeSize) {  // Scales of zero don't do anything.
           // We only allow one variable index at the moment.
           if (VariableOperand != -1)
             return false;