RFR(XXS): 8189244: x86: eliminate frame::adjust_unextended_sp() overhead

[dean.long at oracle.com]

https://bugs.openjdk.java.net/browse/JDK-8189244

http://cr.openjdk.java.net/~dlong/8189244/webrev

The work that frame::adjust_unextended_sp() does is DEBUG_ONLY, but the 
compiler cannot completely eliminate the code because of the virtual 
call to is_compiled() that could have side-effects. We can fix the 
problem by wrapping the whole thing in #ifdef ASSERT.

This change reduces the size of libjvm.so by almost 2K, and the size of 
frame::sender() by 8%.

dl
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