RFR: 8209633: Avoid creating WeakEntry wrappers when looking up cached MethodType

[Claes Redestad]

Hi,

a small improvement can be seen on startup tests that exercise lambdas 
and ISC if we change the interning mechanism of MethodTypes slightly so 
that lookup can be done without wrapping the lookup key in a WeakEntry.

Bug: https://bugs.openjdk.java.net/browse/JDK-8209633
Webrev: http://cr.openjdk.java.net/~redestad/8209633/open.00/

Testing: jdk-tier1+jdk-tier2

Thanks!

/Claes