[BPF] fix indirect call assembly code

Currently, for indirect call, the assembly code printed out as
  callx <imm>
This is not right, it should be
  callx <reg>

Fixed the issue with proper format.

Differential Revision: https://reviews.llvm.org/D69229

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@375386 91177308-0d34-0410-b5e6-96231b3b80d8