RFR: 8278533: Remove some unused methods in c1_Instruction and c1_ValueMap

[Hamlin Li]

This is a trivial patch to remove some unused methods in c1_Instruction and c1_ValueMap.

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Commit messages:
 - Initial commit

Changes: https://git.openjdk.java.net/jdk/pull/6798/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6798&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8278533
  Stats: 8 lines in 3 files changed: 0 ins; 7 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6798.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6798/head:pull/6798

PR: https://git.openjdk.java.net/jdk/pull/6798