[lld] [llvm] [ICF] Add a NOP after branch in ICF thunk to improve debugability (PR #154986)

[Peter Rong via llvm-commits]

DataCorrupted wrote:

Thanks for pointing that out! 

But if you have three consecutive one-instruction functions (which would be a common case if you enable ICF safe thunks), they will be concat into one sequence: https://godbolt.org/z/GhrYP8njv

```
0x0000000000000000      1     12      1   0             0       0  is_stmt prologue_end
0x0000000000000010      2     12      1   0             0       0  is_stmt prologue_end
0x0000000000000020      3     12      1   0             0       0  is_stmt prologue_end
0x0000000000000021      3     12      1   0             0       0  is_stmt end_sequence
```

The problem we have here is, for ICF thunks to be probably symbolicated, each function needs to be its own sequence when we enable `-mllvm -emit-func-debug-line-table-offsets`: https://godbolt.org/z/qbndoGWbM

```
0x0000000000000000      1     12      1   0             0       0  is_stmt prologue_end
0x0000000000000000      1     12      1   0             0       0  is_stmt end_sequence
0x0000000000000010      2     12      1   0             0       0  is_stmt prologue_end
0x0000000000000010      2     12      1   0             0       0  is_stmt end_sequence
0x0000000000000020      3     12      1   0             0       0  is_stmt prologue_end
0x0000000000000021      3     12      1   0             0       0  is_stmt end_sequence
```

In this case, we can just advance PC, as the PC will go into other thunks. 

https://github.com/llvm/llvm-project/pull/154986