bpf: proper print imm64 expression in inst printer

Fixed an issue in printImm64Operand where if the value is
an expression, print out the expression properly. Currently,
it will print
  r1 = <MCOperand Expr:(tx_port)>ll
With the patch, the printout will be
  r1 = tx_port

Suggested-by: Jiong Wang <jiong.wang@netronome.com>
Signed-off-by: Yonghong Song <yhs@fb.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
llvm-svn: 312833