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

[Ludvig Janiuk]

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.

-------------

Commit messages:
 - _hir->verify only after modifying

Changes: https://git.openjdk.java.net/jdk/pull/6456/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6456&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8277382
  Stats: 140 lines in 1 file changed: 24 ins; 29 del; 87 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6456.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6456/head:pull/6456

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