RFR: 8255579: x86: Use cmpq(Register,Address) in safepoint_poll

[Aleksey Shipilev]

JDK-8253180 added a new block in `safepoint_poll` that uses broken `cmpq` (JDK-8255550): it effectively does the comparison with operands swapped. It makes sense to use the non-broken `cmpq`, as to avoid changing the condition code and thus making the code less understandable. See the discussion in #910. 

Testing:
  - [x] `tier1` with Z (some SA tests fail, and some other fail with OOME -- seem to be expected/problem-listed)

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Commit messages:
 - 8255579: x86: Use cmpq(Register,Address) in safepoint_poll

Changes: https://git.openjdk.java.net/jdk/pull/924/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=924&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8255579
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/924.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/924/head:pull/924

PR: https://git.openjdk.java.net/jdk/pull/924