This patch changes the computation of the hash key for DISubprogram to
be computed on a small subset of the fields. The hash is computed a
lot faster, but there might be more collision in the table.
However by carefully selecting the fields, colisions should be rare.
Using opt to load the IR for FastISelEmitter.cpp.o, with this patch:
- DISubprogram::getImpl() goes from 28ms to 15ms.
- DICompositeType::getImpl() goes from 6ms to 2ms
- DIDerivedType::getImpl() goes from 18 to 12ms