* Reimplement most of the logic in Swift as an Instruction simplification and remove the old code from SILCombine
* support more cases of existential archetype replacements:
For example:
```
%0 = alloc_stack $any P
%1 = init_existential_addr %0, $T
use %1
```
is transformed to
```
%0 = alloc_stack $T
use %0
```
Also, if the alloc_stack is already an opened existential and the concrete type is known,
replace it as well:
```
%0 = metatype $@thick T.Type
%1 = init_existential_metatype %0, $@thick any P.Type
%2 = open_existential_metatype %1 : $@thick any P.Type to $@thick (@opened("X", P) Self).Type
...
%3 = alloc_stack $@opened("X", any P) Self
use %3
```
is transformed to
```
...
%3 = alloc_stack $T
use %3
```
Find all the usages of `--enable-experimental-feature` or
`--enable-upcoming-feature` in the tests and replace some of the
`REQUIRES: asserts` to use `REQUIRES: swift-feature-Foo` instead, which
should correctly apply to depending on the asserts/noasserts mode of the
toolchain for each feature.
Remove some comments that talked about enabling asserts since they don't
apply anymore (but I might had miss some).
All this was done with an automated script, so some formatting weirdness
might happen, but I hope I fixed most of those.
There might be some tests that were `REQUIRES: asserts` that might run
in `noasserts` toolchains now. This will normally be because their
feature went from experimental to upcoming/base and the tests were not
updated.
Instead, use the `%target-swift-5.1-abi-triple` substitution to compile the tests
for deployment to the minimum OS versions required for use of _Concurrency APIs.
