[PATCH] [InstCombine] Fix visitSwitchInst to use right operand types for sub cstexpr.

[David Majnemer]

================
Comment at: lib/Transforms/InstCombine/InstructionCombining.cpp:2122
@@ +2121,3 @@
+              TruncCond ? ConstantInt::get(SI.getContext(),
+                                           CaseVal->getValue().sext(BitWidth))
+                        : CaseVal);
----------------
bruno wrote:
> majnemer wrote:
> > Is there any particular reason why you chose to sext instead of zext?
> Being conservative to perform the right extension when coming back from the truncated negative condition cases. Would that be somehow redundant in this scenario?
I don't think it will actually be faster.

`ConstantExpr::getSExt(CaseVal, Cond->getType())` will call `getFoldedCast` which will call `ConstantFoldCastInstruction` which will do the following:
   case Instruction::SExt:
     if (ConstantInt *CI = dyn_cast<ConstantInt>(V)) {
       uint32_t BitWidth = cast<IntegerType>(DestTy)->getBitWidth();
       return ConstantInt::get(V->getContext(),
                               CI->getValue().sext(BitWidth));
     }

Seems like it will do the exact same thing.  No table lookups involved.

http://reviews.llvm.org/D6644

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