RFR: 8263672: fatal error: no reachable node should have no use

[Roland Westrelin]

Follow up to 8263577 (C2: reachable nodes shouldn't have dead uses at
the end of optimizations), a case I missed where set_req needs to be
replaced by set_req_X.

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

Commit messages:
 - fix

Changes: https://git.openjdk.java.net/jdk/pull/3043/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3043&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8263672
  Stats: 7 lines in 1 file changed: 0 ins; 5 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3043.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3043/head:pull/3043

PR: https://git.openjdk.java.net/jdk/pull/3043