RFR: 8257057: C2: Improve safepoint processing during vector scalarization pass

[Vladimir Ivanov]

Cast nodes (`CheckCastPP`/`CastPP`) hinders scalarization of vectors since they aren't taken into account when affected safepoints are enumerated.

Proposed fix implements a reversed variant of `Node::uncast()` to find all safepoints which have vectors referenced from their debug info and then uses `Node::uncast()` when iterating over debug edges.  It is safe to ignore cast nodes (even the ones which carry control dependency): `VectorBox` already contains the most precise type information and the vector value it represents is immutable. So, it's safe to replace a fully constructed boxed vector instance with the vector value it contains and rematerialize the equivalent box instance if deoptimization happens.    

Testing:
- `jdk/incubator/vector` tests w/ different flag combinations (no flags, `-Xcomp`, `-XX:+DeoptimizeALot`); 
- tier1-4

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Commit messages:
 - Improve safepoint processing during vector scalarization pass.

Changes: https://git.openjdk.java.net/jdk/pull/1456/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1456&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8257057
  Stats: 20 lines in 1 file changed: 9 ins; 0 del; 11 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1456.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1456/head:pull/1456

PR: https://git.openjdk.java.net/jdk/pull/1456