[analyzer] Optimize constraint generation when the range is a concrete value

Summary:
If a constraint is something like:
```
$0 = [1,1]
```
it'll now be created as:
```
assert($0 == 1)
```
instead of:
```
assert($0 >= 1 && $0 <= 1)
```

In general, ~3% speedup when solving per query in my machine. Biggest improvement was when verifying sqlite3, total time went down from 3000s to 2200s.

I couldn't create a test for this as there is no way to dump the formula yet. D48221 adds a method to dump the formula but there is no way to do it from the command line.

Also, a test that prints the formula will most likely fail in the future, as different solvers print the formula in different formats.

Reviewers: NoQ, george.karpenkov, ddcc

Reviewed By: george.karpenkov

Subscribers: xazax.hun, szepet, a.sidorin

Differential Revision: https://reviews.llvm.org/D48227

llvm-svn: 335116