RFR: 8366076: arm32: Fix register allocation for vector instructions

[Max Verevkin]

Arm32 has 32 double-precision floating point registers, the first 16 of which coincide with the 32 single-precision floating point registers. Some vector-operation nodes were implemented in terms of scalar instructions, which only really works for the first 16 doubles. This commit addresses that.

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Commit messages:
 - 8366076: arm32: Fix register allocation for vector instructions

Changes: https://git.openjdk.org/jdk/pull/27071/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27071&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8366076
  Stats: 31 lines in 2 files changed: 25 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/27071.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27071/head:pull/27071

PR: https://git.openjdk.org/jdk/pull/27071