Request for reviews (S): 7080431: VM asserts if specified size(x) in .ad is larger than emitted size

[Vladimir Kozlov]

http://cr.openjdk.java.net/~kvn/7080431/webrev

7080431: VM asserts if specified size(x) in .ad is larger than emitted size

It was allowed to specify larger size(x) in mach node definition in .ad file 
than actual emitted instruction size. It was treated as upper bound on 
instruction size. 7063629 changes broke that, it requires size(x) in mach node 
definition match the emitted size which reduced flexibility in C2 development.

Move code from finalize_offsets_and_shorten() to fill_buffer() to restore 
previous behavior.