[16] RFR(S): 8251525: AARCH64: Faster Math.signum(fp)

[Dmitry Chuyko]

Hello,

Please review a faster version of Math.signum() for AArch64.

Two new intrinsics (double and float) are introduced in general code, 
with appropriate new nodes. New JTreg test is added to cover the 
intrinsic case (enabled only for aarch64).

AArch64 implementation uses FACGT (compare abslute fp values) and BSL 
(fp bit selection) to avoid branches and moves to non-fp registers and back.

Performance results show ~30% better time in the benchmark with a black 
hole [1] on Cortex. E.g. on random numbers 4.8 ns/op --> 3.5 ns/op, 
overhead is 2.9 ns/op.

rfe: https://bugs.openjdk.java.net/browse/JDK-8251525
webrev: http://cr.openjdk.java.net/~dchuyko/8251525/webrev.00/
testing: jck, jtreg including new dedicated test

-Dmitry

[1] https://cr.openjdk.java.net/~dchuyko/8249198/DoubleSignum.java