RFR: 8253002: Remove the unused SafePointNode::_oop_map field

[Jorn Vernee]

Hi,

I've been looking a lot at the code for generating oop maps for call nodes lately, and noticed that SafePointNode had
an oopMap field that was unused (which led to some confusion as to where the oop map was actually set).

The oop map is instead generated and set in buildOopMap OopFlow::compute_reach after matching:
https://github.com/openjdk/jdk/blob/master/src/hotspot/share/opto/buildOopMap.cpp#L122 So the field on the ideal node
is unused.

This patch removes the field and cleans up related code. I've left a comment in SafePointNode to point people looking
for the oop map at buildOopMap.cpp

Thanks,
Jorn

Testing: local build + tier1,tier2,tier3

-------------

Commit messages:
 - Remove the unused SafePointNode::_oop_map field

Changes: https://git.openjdk.java.net/jdk/pull/109/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=109&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253002
  Stats: 8 lines in 2 files changed: 1 ins; 6 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/109.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/109/head:pull/109

PR: https://git.openjdk.java.net/jdk/pull/109