RFR: 8251926: PPC: Remove an unused variable in assembler_ppc.cpp

[Michihiro Horie]

Dear all,

Would you please review a small change?

Bug: https://bugs.openjdk.java.net/browse/JDK-8251926
Webrev: http://cr.openjdk.java.net/~mhorie/8251926/webrev.00/

The load_const_optimized function in assembler_ppc.cpp has an unused
variable named return_xd. It looks unnecessary in the current code.

Best regards,
Michihiro