cmd/compile: fix disassembly of invalid instructions

cmd/compile: fix disassembly of invalid instructions

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/37255
Reviewed-by: Cherry Zhang <cherryyz@google.com>