Make sure that if we encode an explicit base register, we print it.
That will ensure that if we make an Addr with an auto variable but
a base that isn't SP, then it will be obvious from the disassembly.

Update #19184

Change-Id: If5556a5183f344d719ec7197aa935a0166061e6f
Reviewed-on: https://go-review.googlesource.com/37255Reviewed-by: Cherry Zhang <cherryyz@google.com>