RFR: 8342975: C2: Micro-optimize PhaseIdealLoop::Dominators()

[Aleksey Shipilev]

Noticed this while looking at Leyden profiles. C2 seems to spend considerable time doing in this loop. The disassembly shows this loop is fairly hot. Replacing the initialization with memset, while touching more memory, is apparently faster. memset is also what we normally do around C2 for arena-allocated data. We seem to touch a lot of these structs later on, so pulling them to cache with memset is likely "free". 

It also looks like current initialization misses initializing the last element (at `C->unique()+1`).

I'll put performance data in separate comment.

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/21690/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21690&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8342975
  Stats: 5 lines in 1 file changed: 1 ins; 1 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/21690.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21690/head:pull/21690

PR: https://git.openjdk.org/jdk/pull/21690