[riscv-port] RFR: 8278192: riscv: remove unnecessary instruct of DecodeNKlass in C2

[Yadong Wang]

There are two instructs for DecodeNKlass in C2, and the difference is only that which temporary register operand is used by them, xheapbase, t0 or t1. It's too complicated and the effect is almost invisible. So we just simplify the pattern by using a temporary register from the register allocation.

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Commit messages:
 - 8278192: riscv: remove unnecessary instruct of DecodeNKlass in C2

Changes: https://git.openjdk.java.net/riscv-port/pull/22/files
 Webrev: https://webrevs.openjdk.java.net/?repo=riscv-port&pr=22&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8278192
  Stats: 51 lines in 3 files changed: 3 ins; 30 del; 18 mod
  Patch: https://git.openjdk.java.net/riscv-port/pull/22.diff
  Fetch: git fetch https://git.openjdk.java.net/riscv-port pull/22/head:pull/22

PR: https://git.openjdk.java.net/riscv-port/pull/22