RFR (XS): 7102300: performance warnings cause results diff failure in Test6890943

[David Chase]

http://cr.openjdk.java.net/~drchase/7102300/webrev.00/

bug:
compiler/5091921/Test6890943.java sometimes tickles a hashtable
performance warning, unrelated to what it is supposed to be testing.
This warning causes a diff against golden results, which is classified as a failure.

fix:
(1) Strip lines matching the performance warning from the output before diff.
(2) Add -XX+IgnoreUnrecognizedVMOptions to avoid other failure-generating warnings.

testing: 
JTreg, JPRT.