RFR: 8203352: Improve java implementation of Integer/Long.numberOfLeadingZeros

[Claes Redestad]

Hi,

while there are C2 intrinsics on most platforms providing access to 
specialized hardware instructions, e.g., lzcnt on Intel, optimizing the 
java implementations of Integer/Long.numberOfLeadingZeros can still be 
worthwhile, especially if it also helps C1 and implicitly 
startup/warmup. This implementation wins slightly (5-25%) over the 
baseline in all tested optimization modes (-Xint, 
-XX:TieredStopAtLevel=1-3), as well as on C2 if the intrinsics are disabled.

Webrev: http://cr.openjdk.java.net/~redestad/8203352/open.00/

Bug: https://bugs.openjdk.java.net/browse/JDK-8203352

Correctness is checked by existing tests, mainly 
test/jdk/java/lang/Integer|Long/BitTwiddle.java

/Claes