Request for reviews (M): 6710487: More than half of JDI Regression tests hang with COOPs in -Xcomp mode

[Vladimir Kozlov]

I updated webrev with new code.

http://webrev.invokedynamic.info/kvn/6710487/index.html

Fixed 6710487: More than half of JDI Regression tests hang with COOPs in -Xcomp mode

Problem:
DecodeNNode::decode() and EncodePNode::encode() contains
Identity transformation (may return a preexisting ideal node)
which could cause infinite loop during gvn transformation.

Solution:
Remove DecodeNNode::decode() and EncodePNode::encode() methods.
Both nodes now have Identity() and Value() methods which
do transformations we need.

Replace TypeOopPtr::make_narrowoop() with Type::make_narrowoop() and
TypeNarrowOop::make_oopptr() with Type::make_ptr() to simplify the code.

Reviewed by:
Fix verified (y/n): y, all JDI regression tests

Other testing:
JPRT, CTW, NSK, runThese