[Attributor] Use a pointer value type for the OpcodeInstMap

This reduces memory consumption and the need to copy complex data
structures repeatedly.

No functional change is intended.

---

Single run of the Attributor module and then CGSCC pass (oldPM)
for SPASS/clause.c (~10k LLVM-IR loc):

Before:
```
calls to allocation functions: 490390 (320725/s)
temporary memory allocations: 84601 (55330/s)
peak heap memory consumption: 41.70MB
peak RSS (including heaptrack overhead): 131.18MB
total memory leaked: 269.04KB
```

After:
```
calls to allocation functions: 489359 (301144/s)
temporary memory allocations: 82983 (51066/s)
peak heap memory consumption: 36.76MB
peak RSS (including heaptrack overhead): 126.48MB
total memory leaked: 269.04KB
```

Difference:
```
calls to allocation functions: -1031 (-10739/s)
temporary memory allocations: -1618 (-16854/s)
peak heap memory consumption: -4.94MB
peak RSS (including heaptrack overhead): 0B
total memory leaked: 0B

---