[BPF] Prevent disassembly segfault for NOP insn

For a simple program like below:
  -bash-4.4$ cat t.c
  int test() {
    asm volatile("r0 = r0" ::);
    return 0;
  }
compiled with
  clang -target bpf -O2 -c t.c
the following llvm-objdump command will segfault.
  llvm-objdump -d t.o

  0:       bf 00 00 00 00 00 00 00 nop
  llvm-objdump: ../include/llvm/ADT/SmallVector.h:180
  ...
  Assertion `idx < size()' failed
  ...
  abort
  ...
  llvm::BPFInstPrinter::printOperand
  llvm::BPFInstPrinter::printInstruction
  ...

The reason is both NOP and MOV_rr (r0 = r0) having the same encoding.
The disassembly getInstruction() decodes to be a NOP instruciton but
during printInstruction() the same encoding is interpreted as
a MOV_rr instruction. Such a mismatcch caused the segfault.

The fix is to make NOP instruction as CodeGen only so disassembler
will skip NOP insn for disassembling.

Note that instruction "r0 = r0" should not appear in non inline
asm codes since BPF Machine Instruction Peephole optimization will
remove it.

Differential Revision: https://reviews.llvm.org/D80156

llvm/lib/Target/BPF/BPFInstrInfo.td

@@ -526,7 +526,7 @@ class NOP_I<string OpcodeStr>
   let BPFClass = BPF_ALU64;
 }
 
-let hasSideEffects = 0 in
+let hasSideEffects = 0, isCodeGenOnly = 1 in
   def NOP : NOP_I<"nop">;
 
 class RET<string OpcodeStr>

llvm/test/CodeGen/BPF/objdump_nop.ll

@@ -0,0 +1,19 @@
+; RUN: llc -march=bpfel -filetype=obj -o - %s | llvm-objdump -d - | FileCheck %s
+;
+; Source:
+;   int test() {
+;     asm volatile("r0 = r0" ::);
+;     return 0;
+;   }
+; Compilation flag:
+;   clang -target bpf -O2 -S -emit-llvm t.c
+
+; Function Attrs: nounwind
+define dso_local i32 @test() local_unnamed_addr {
+entry:
+  tail call void asm sideeffect "r0 = r0", ""()
+  ret i32 0
+}
+; CHECK-LABEL: test
+; CHECK:       r0 = r0
+; CHECK:       r0 = 0