RFR: 8272791: java -XX:BlockZeroingLowLimit=1 crashes after 8270947

[Andrew Haley]

This is an assertion failure caused by setting `BlockZeroingLowLimit` < `wordSize`.

I believe there was some confusion when writing this code about whether `BlockZeroingLowLimit` should be in words or bytes, and it makes no sense for it to be less than a single word. If anyone ever tried to use a parameter < 8, it triggers an assertion.

This patch does two things. Firstly, it corrects a `guarantee` which erroneously used `zero_words_block_size` rather than `wordSize`. The value of both of these is 8, so it doesn't change anything in the generated code. Secondly, it clips the `BlockZeroingLowLimit` so that it doesn't trigger the assertion.

Perhaps it would be better to change the lower limit to 8 instead of this silent correction. There is no backward compatibility issue here, because any attempt to set `BlockZeroingLowLimit` < 8 in the past would have exited the VM with an error, so I don't believe a CSR is warranted if we do change the allowable range.

So, which should it be? Change the lower limit of the range of the `BlockZeroingLowLimit` system flag, or allow 1 still to be used and silently fix it? Opinions welcome.

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Commit messages:
 - 8272791: java -XX:BlockZeroingLowLimit=1 crashes after 8270947

Changes: https://git.openjdk.java.net/jdk/pull/8756/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8756&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8272791
  Stats: 7 lines in 2 files changed: 6 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8756.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8756/head:pull/8756

PR: https://git.openjdk.java.net/jdk/pull/8756