RFR: 8240615: is_power_of_2() has Undefined Behaviour and is inconsistent

[Andrew Haley]

So, my plan is to push http://cr.openjdk.java.net/~aph/8240615-1/ and
subsequently patch a few AArch64 cases plus the x86_64 immL_Pow2 and
immL_NotPow2 cases that we already identified.

I do not intend to do bulk changes to any other clients of
is_power_of_2() because it's often unclear exactly what behaviour is
required, and I'd risk breaking something.

OK?

-- 
Andrew Haley  (he/him)
Java Platform Lead Engineer
Red Hat UK Ltd. <https://www.redhat.com>
https://keybase.io/andrewhaley
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