[Attributor] KindToAbstractAttributeMap: use SmallDenseMap

[lebedev.ri]

Summary

While this is less efficient to allocate huge SmallDenseMap for each IRPosition in AAMap,
in the larger picture this is much better, since we'd eventually either fill each IRPosition,
with each possible attribute, or at least quert for it, which would allocate it anyway.
So we are better off pre-allocating.

Old:

0.3460 ( 40.7%)   0.0183 ( 33.9%)   0.3643 ( 40.3%)   0.3644 ( 40.3%)  Deduce and propagate attributes (CGSCC pass)
0.1135 ( 13.4%)   0.0080 ( 14.7%)   0.1215 ( 13.4%)   0.1215 ( 13.4%)  Deduce and propagate attributes

total runtime: 19.48s.
bytes allocated in total (ignoring deallocations): 575.02MB (29.51MB/s)
calls to allocation functions: 908876 (46644/s)
temporary memory allocations: 276654 (14198/s)
peak heap memory consumption: 26.68MB
peak RSS (including heaptrack overhead): 944.78MB
total memory leaked: 8.85MB

New:

0.3223 ( 38.1%)   0.0299 ( 53.6%)   0.3522 ( 39.1%)   0.3522 ( 39.1%)  Deduce and propagate attributes (CGSCC pass)
0.1150 ( 13.6%)   0.0037 (  6.7%)   0.1188 ( 13.2%)   0.1188 ( 13.2%)  Deduce and propagate attributes

total runtime: 19.06s.
bytes allocated in total (ignoring deallocations): 363.21MB (19.06MB/s)
calls to allocation functions: 679660 (35658/s)
temporary memory allocations: 83472 (4379/s)
peak heap memory consumption: 27.00MB
peak RSS (including heaptrack overhead): 931.66MB
total memory leaked: 8.85MB

Diff:

total runtime: -0.42s.
bytes allocated in total (ignoring deallocations): -211.81MB (498.38MB/s)
calls to allocation functions: -229216 (539331/s)
temporary memory allocations: -193182 (454545/s)
peak heap memory consumption: 321.54KB
peak RSS (including heaptrack overhead): 0B
total memory leaked: 0B

[jdoerfert]

LGTM. Thx!

llvm/include/llvm/Transforms/IPO/Attributor.h

private:
   AAVector AllAbstractAttributes;
   ///}
 
   /// A nested map to lookup abstract attributes based on the argument position
   /// on the outer level, and the addresses of the static member (AAType::ID) on
   /// the inner level.
   ///{
   using KindToAbstractAttributeMap =
-      DenseMap<const char *, AbstractAttribute *>;
+      SmallDenseMap<const char *, AbstractAttribute *, /*InlineBuckets=*/32>;
   DenseMap<IRPosition, KindToAbstractAttributeMap> AAMap;
   ///}
 
   /// A map from abstract attributes to the ones that queried them through calls
   /// to the getAAFor<...>(...) method.
   ///{
   struct QueryMapValueTy {
     /// Set of abstract attributes which were used but not necessarily required