[BPF] explicit warning of not supporting dynamic stack allocation

[yonghong-song]

Summary

Currently, BPF does not support dynamic static allocation.
For a program like below:

extern void bar(int *); 
void foo(int n) {
  int a[n];
  bar(a);
}

The current error message looks like:

unimplemented operand
UNREACHABLE executed at /.../llvm/lib/Target/BPF/BPFISelLowering.cpp:199!

Let us make error message explicit so it will be clear to the user
what is the problem. With this patch, the error message looks like:

fatal error: error in backend: Unsupported dynamic stack allocation
...

llvm/lib/Target/BPF/BPFISelLowering.cpp

 SDValue BPFTargetLowering::LowerOperation(SDValue Op, SelectionDAG &DAG) const {
   switch (Op.getOpcode()) {
   case ISD::BR_CC:
     return LowerBR_CC(Op, DAG);
   case ISD::GlobalAddress:
     return LowerGlobalAddress(Op, DAG);
   case ISD::SELECT_CC:
     return LowerSELECT_CC(Op, DAG);
+  case ISD::DYNAMIC_STACKALLOC:
+    report_fatal_error("Unsupported dynamic stack allocation");
   default:
     llvm_unreachable("unimplemented operand");
   }
 }
 
 // Calling Convention Implementation
 #include "BPFGenCallingConv.inc"