RFR: JDK-8277382 make c1 BlockMerger use IR::verify only when necessary

[Ludvig Janiuk]

On Thu, 18 Nov 2021 13:30:02 GMT, Ludvig Janiuk <duke at openjdk.java.net> wrote:

> This PR removes two calls to `IR::verify` which were unnecessary. The reason they are unnecessary is that `try_merge` does not always take any action. There is not need to verify if nothing has changed. In the cases that `try_merge` does do anything, it already calls `IR::verify` afterwards.
> 
> This PR also switches some deeply nested if statements in `try_merge` to early returns.

Passes both tier1 and tier2 in debug build.

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PR: https://git.openjdk.java.net/jdk/pull/6456