[BOLT] stale profile matching [part 2 out of 2]

This is a first "serious" version of stale profile matching in BOLT. This diff
extends the hash computation for basic blocks so that we can apply a fuzzy
hash-based matching. The idea is to compute several "versions" of a hash value
for a basic block. A loose version of a hash (computed by ignoring instruction
operands) allows to match blocks in functions whose content has been changed,
while stricter hash values (considering instruction opcodes with operands and
even based on hashes of block's successors/predecessors) allow to resolve
collisions. In order to save space and build time, individual hash components
are blended into a single uint64_t.
There are likely numerous ways of improving hash computation but already this
simple variant provides significant perf benefits.

**Perf testing** on the clang binary: collecting data on clang-10 and using it
to optimize clang-11 (with ~1 year of commits in between). Next, we compare
- //stale_clang// (clang-11 optimized with profile collected on clang-10 with **infer-stale-profile=0**)
- //opt_clang// (clang-11 optimized with profile collected on clang-11)
- //infer_clang// (clang-11 optimized with profile collected on clang-10 with **infer-stale-profile=1**)

`LTO-only` mode:
//stale_clang// vs //opt_clang//: task-clock [delta(%): 9.4252 ± 1.6582, p-value: 0.000002]
(That is, there is a ~9.5% perf regression)
//infer_clang// vs //opt_clang//: task-clock [delta(%): 2.1834 ± 1.8158, p-value: 0.040702]
(That is, the regression is reduced to ~2%)
Related BOLT logs:
```
BOLT-INFO: identified 2114 (18.61%) stale functions responsible for 30.96% samples
BOLT-INFO: inferred profile for 2101 (18.52% of all profiled) functions responsible for 30.95% samples
```

`LTO+AutoFDO` mode:
//stale_clang// vs //opt_clang//: task-clock [delta(%): 19.1293 ± 1.4131, p-value: 0.000002]
//infer_clang// vs //opt_clang//: task-clock [delta(%): 7.4364 ± 1.3343, p-value: 0.000002]
Related BOLT logs:
```
BOLT-INFO: identified 5452 (50.27%) stale functions responsible for 85.34% samples
BOLT-INFO: inferred profile for 5442 (50.23% of all profiled) functions responsible for 85.33% samples
```

Reviewed By: Amir

Differential Revision: https://reviews.llvm.org/D146661