Handle big index in getelementptr instruction

CodeGen incorrectly ignores (assert from APInt) constant index bigger
than 2^64 in getelementptr instruction. This is a test and fix for that.

Patch by Paweł Bylica!

Reviewed By: rnk

Subscribers: majnemer, rnk, mcrosier, resistor, llvm-commits

Differential Revision: http://reviews.llvm.org/D8219

llvm-svn: 231984
diff --git a/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp b/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp
index faa36b0..16724b0 100644
--- a/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp
@@ -497,7 +497,7 @@
        OI != E; ++OI) {
     const Value *Idx = *OI;
     if (auto *StTy = dyn_cast<StructType>(Ty)) {
-      unsigned Field = cast<ConstantInt>(Idx)->getZExtValue();
+      uint64_t Field = cast<ConstantInt>(Idx)->getZExtValue();
       if (Field) {
         // N = N + Offset
         TotalOffs += DL.getStructLayout(StTy)->getElementOffset(Field);
@@ -518,8 +518,8 @@
         if (CI->isZero())
           continue;
         // N = N + Offset
-        TotalOffs +=
-            DL.getTypeAllocSize(Ty) * cast<ConstantInt>(CI)->getSExtValue();
+        uint64_t IdxN = CI->getValue().sextOrTrunc(64).getSExtValue();
+        TotalOffs += DL.getTypeAllocSize(Ty) * IdxN;
         if (TotalOffs >= MaxOffs) {
           N = fastEmit_ri_(VT, ISD::ADD, N, NIsKill, TotalOffs, VT);
           if (!N) // Unhandled operand. Halt "fast" selection and bail.