RFR(S): 8063086: Math.pow yields different results upon repeated calls

[Roland Westrelin]

http://cr.openjdk.java.net/~roland/8063086/webrev.00/

With 8029302, C2 computes x^2 as x*x. The interpreter and C1 code don’t have the special case code and as a result an application can see different result for the same computation whether it’s executed by the interpreter/c1 or c2. Fixed by adding the special case to the interpreter and C1.

Roland.