RFR(S) 8010281: Remove code that never execute in opto/ifg.cpp

[Niclas Adlertz]

Hi all.

Problem:
In the C2 register allocator, the define EXACT_PRESSURE is set to decide how we estimate the block pressure. There are currently two different ways on how to estimate this. However, EXACT_PRESSURE is always set to 1. This means that the code for the second choice on how to estimate block pressure is never executed, and therefore not needed. 

Solution:
Remove the define EXACT_PRESSURE and the code that is never executed; executing only the code that now executes within the ifdefs of EXACT_PRESSURE.

A review would be much appreciated. 
Thank you.

Regards,
Niclas Adlertz

---------------------

WEBREV: http://cr.openjdk.java.net/~neliasso/8010281/webrev00/
BUG: https://jbs.oracle.com/bugs/browse/JDK-8010281, http://bugs.sun.com/view_bug.do?bug_id= 8010281