RFR: JDK-8304539: Cleanup utilities/{count_leading_zeros, count_trailing_zeros, population_count}.hpp [v13]

[Justin King]

> As the title says, cleanup the mentioned headers. This is similar to `byteswap.hpp` and removes the extraneous `#ifdef` for XLC since it is really just Clang now.

Justin King has updated the pull request incrementally with one additional commit since the last revision:

  Update comment
  
  Signed-off-by: Justin King <jcking at google.com>

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Changes:
  - all: https://git.openjdk.org/jdk/pull/13103/files
  - new: https://git.openjdk.org/jdk/pull/13103/files/668fb06e..e52ce442

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13103&range=12
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13103&range=11-12

  Stats: 5 lines in 1 file changed: 0 ins; 1 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/13103.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13103/head:pull/13103

PR: https://git.openjdk.org/jdk/pull/13103