Fix ExistentialSpecializer: Recursive function specialization

[rajbarik]

This PR fixes a bug in ExistentialSpecializer where a recursive function when specialized crashed the compiler. I have attached a test case with the PR.

[rajbarik]

@atrick can you accept this PR?
A performance benchmark is almost done for enabling ES by default (#19820).

[atrick]

The change seems fine. I want to set better precedent with the style of test though. The biggest difficulty in changing SIL today is knowing how to update all the CHECK lines without understanding the original intention. I think this should be a .sil test and it needs a comment to explain the bailout that it exercises.

[rajbarik]

@atrick I added a sil test case. the primary goal for both swift and sil test cases are that they check for signatures and hence function_ref instructions are fully checked.

Please review when you get a chance.

[rajbarik]

@atrick please run tests. I have addressed all your concerns.

[atrick]

@swift-ci test.

[atrick]

@rajbarik is the "Stable with ES in open source" commit supposed to be in this PR. I don't remember reviewing that.

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 70ac1b2fb8aae27dcb9ed96c7a1b11f21461d61e

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 70ac1b2fb8aae27dcb9ed96c7a1b11f21461d61e

[rajbarik]

@rajbarik is the "Stable with ES in open source" commit supposed to be in this PR. I don't remember reviewing that.

Some git mess. Let me fix this.

[rajbarik]

@atrick fixed.

[rajbarik]

@atrick please run tests here again. Thanks.

[atrick]

@swift-ci test.

[rajbarik]

@atrick any idea why this is not reporting anything for last 3 days?

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 6d9f532

[rajbarik]

@atrick please advise. I do not any logs for Swift Test Linux Platform.

[atrick]

@swift-ci test linux platform.