RFR: backport for fix https://bugs.openjdk.java.net/browse/JDK-8172291

[Maurizio Cimadamore]

Hi,
please review the backport:

https://bugs.openjdk.java.net/browse/JDK-8172291

The original issue is here (fixed in 9):

https://bugs.openjdk.java.net/browse/JDK-8168774

The JDK 9 patch does not apply cleanly because:

* Types.isSignaturePolymorphic has changed in 9 because of VarHandles
* the test (BadPolySig) contains jtreg module directives which are not 
valid in 8

Webrev here:

http://cr.openjdk.java.net/~mcimadamore/8172291/

Cheers
Maurizio