RFR (S): 8227671: G1: assert_used_and_recalculate_used_equal performs work in product builds

[Thomas Schatzl]

Hi all,

  can I have reviews for this small change that avoids doing
unneccessary work in product builds?

In particular, the assert_used_and_recalculate_used_equal macro
executes code (the recalculate_used() call) that is potentially
lengthy.

I measured a few ms on a large heap (50GB) with (forced) 50k regions. I
am not sure why this call is so expensive in my tests, but it is
completely unnecessary.

CR:
https://bugs.openjdk.java.net/browse/JDK-8227671
Webrev:
http://cr.openjdk.java.net/~tschatzl/8227671/webrev/
Testing:
Currently running hs-tier1-3, local gc/g1 jtreg run, local benchmarks

Thanks,
  Thomas