RFR: 8283257: x86: Clean up invocation/branch counter updates code

[Aleksey Shipilev]

I looked briefly at optimizing `InterpreterMacroAssembler::increment_mask_and_jump` a bit, but it looks that current code is the best we can do. This improvement does a few related cleanups without semantic changes. 

Additional testing:
 - [x] Linux x86_64 fastdebug `tier1`
 - [x] Eyeballing interpreter generated code

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

Commit messages:
 - Cleanups

Changes: https://git.openjdk.java.net/jdk/pull/7838/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7838&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8283257
  Stats: 22 lines in 4 files changed: 0 ins; 6 del; 16 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7838.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7838/head:pull/7838

PR: https://git.openjdk.java.net/jdk/pull/7838