[GlobalISel] Don't add duplicate successors to MBBs when translating indirectbr

This fixes a verifier failure on a bot:

http://green.lab.llvm.org/green/job/test-suite-verify-machineinstrs-aarch64-O0-g/

```
*** Bad machine code: MBB has duplicate entries in its successor list. ***
- function:    foo
- basic block: %bb.5 indirectgoto (0x7fe3d687ca08)
```

One of the GCC torture suite tests (pr70460.c) has an indirectbr instruction
which has duplicate blocks in its destination list.

According to the langref this is allowed:

> Blocks are allowed to occur multiple times in the destination list, though
> this isn’t particularly useful.
(https://www.llvm.org/docs/LangRef.html#indirectbr-instruction)

We don't allow this in MIR. So, when we translate such an instruction, the
verifier screams.

This patch makes `translateIndirectBr` check if a successor has already been
added to a block. If the successor is present, it is skipped rather than added
twice.

Differential Revision: https://reviews.llvm.org/D79609