RFR: 8233159: Method::result_type should use calculated value in constMethod

[Claes Redestad]

Hi,

a simple, independent optimization extracted from the changes proposed
in JDK-8230199[1]:

Webrev: http://cr.openjdk.java.net/~redestad/8233159/open.00/
Bug:    https://bugs.openjdk.java.net/browse/JDK-8233159

Method::result_type is showing up in various places in profiles, and
while non of those places are very hot, the optimization is trivial and
effecient (turns calls that retire 2-400 instructions into an inlined
memory read).

The patch also removes a redundant iterate() call in the related
Bytecode_member_ref::result_type() method (rts.type() will trigger
a scan "lazily").

Testing: tier1-3

Thanks!

/Claes

[1] https://bugs.openjdk.java.net/browse/JDK-8230199