RFR: JDK-8277382 make c1 BlockMerger use IR::verify only when necessary [v2]

[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.

Ludvig Janiuk has updated the pull request incrementally with one additional commit since the last revision:

  indentation error fixed

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/6456/files
  - new: https://git.openjdk.java.net/jdk/pull/6456/files/61d7be5e..e17460c6

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=6456&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=6456&range=00-01

  Stats: 4 lines in 1 file changed: 0 ins; 0 del; 4 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