[MLIR] Remove constexpr from LoopOps.td

[nmostafa]

Summary

Make gcc 5 happy

[dcaballe]

We are committing this since the change is trivial and it's breaking a build.

[rriddle]

Feel free to submit a small+simple fix like this directly next time.

mlir/include/mlir/Dialect/LoopOps/LoopOps.td

let extraClassDeclaration = [{
     void setUpperBound(Value bound) { getOperation()->setOperand(1, bound); }
     void setStep(Value step) { getOperation()->setOperand(2, step); }
     
     /// Number of region arguments for loop-carried values
     unsigned getNumRegionIterArgs() {
       return getBody()->getNumArguments() - 1; 
     }
     /// Number of operands controlling the loop: lb, ub, step
-    constexpr unsigned getNumControlOperands() { return 3; }
+    unsigned getNumControlOperands() { return 3; }
     /// Does the operation hold operands for loop-carried values
     bool hasIterOperands() { 
       return getOperation()->getNumOperands() > getNumControlOperands();
     }
     /// Get Number of loop-carried values
     unsigned getNumIterOperands() {
       return getOperation()->getNumOperands() - getNumControlOperands();
     }