[Attributor] KindToAbstractAttributeMap: use SmallDenseMap

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
```

Reviewers: jdoerfert, sstefan1, uenoku

Reviewed By: jdoerfert

Subscribers: uenoku, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D78231

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

@@ -1207,7 +1207,7 @@ private:
   /// the inner level.
   ///{
   using KindToAbstractAttributeMap =
-      DenseMap<const char *, AbstractAttribute *>;
+      SmallDenseMap<const char *, AbstractAttribute *, /*InlineBuckets=*/32>;
   DenseMap<IRPosition, KindToAbstractAttributeMap> AAMap;
   ///}