RFR: 8350480: RISC-V: Relax assertion about registers in C2_MacroAssembler::minmax_fp

[Fei Yang]

Hi, please review this trivial change.
The current assersion about input registers is more than needed.
It requires that `dst`, `src1` and `src2` must be different from each other.
But the code only required that `dst` must be different from `src1` and `src2`.
Patch simply relaxed the assersion removing the unneeded constraint.

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Commit messages:
 - 8350480: RISC-V: Relax assertion about registers in C2_MacroAssembler::minmax_fp

Changes: https://git.openjdk.org/jdk/pull/23723/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23723&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8350480
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/23723.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23723/head:pull/23723

PR: https://git.openjdk.org/jdk/pull/23723