Code review request: 6604422: G1: re-use half-promoted regions (S)

[Tony Printezis]

Ensures that the last region allocated to during one GC is re-used, when 
possible, during the next GC and it's not left half-full.

http://cr.openjdk.java.net/~tonyp/6604422/webrev.0/

Tony

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| Tony Printezis, Staff Engineer    | Sun Microsystems Inc.          |
|                                   | MS BUR02-311                   |
| e-mail: tony.printezis at sun.com    | 35 Network Drive               |
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