RFR: 8279057: Consolidate InstructionPrinter::do_BlockBegin and remove extra lines in logs of blocks with phi functions

[Hamlin Li]

This pr does following things:
 - First, the code related to printing phi in InstructionPrinter::do_BlockBegin is bit redudant, it could be consolidated;
 - Second, only locals and stack values related to phi functions are printed out now.
 - Third, there are extra blank lines in printed log of blocks with phi functions, these blank lines are better to be removed.

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Commit messages:
 - Refine comments and messages
 - Initial commit

Changes: https://git.openjdk.java.net/jdk/pull/6921/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6921&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8279057
  Stats: 59 lines in 1 file changed: 7 ins; 25 del; 27 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6921.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6921/head:pull/6921

PR: https://git.openjdk.java.net/jdk/pull/6921