RFR: 8294261: AArch64: Use pReg instead of pRegGov when possible

[Ningsheng Jian]

Currently we allocate SVE predicate register p0-p6 for pRegGov operand, which are used as governing predicates for load/store and arithmetic, and also define pReg operand for all allocatable predicate registers. Since some SVE instructions are fine to use/define p8-p15, e.g. predicate operations, this patch makes the matcher work for mixed use of pRegGov and pReg, and tries to match pReg when possible. If a predicate reg is defined as pReg but used as pRegGov, register allocator will handle that properly.

With p8-p15 being used as non-temp register, we need to save them as well when saving all registers. The code of setting predicate reg slot in OopMap in RegisterSaver::save_live_registers() is also removed, because on safepoint, vector masks have been transformed to vector [1].

Tested on different SVE systems. Also tested with making RA to allocate p8-p15 first for vReg operand, so that a p8-p15 reg has more chance to be allocated, and if an SVE instruction, emitted by ad rule, does not accept p8-p5, assembler will crash.

[1] https://github.com/openjdk/jdk/blob/master/src/hotspot/share/opto/vector.cpp#L265

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Commit messages:
 - 8294261: AArch64: Use pReg instead of pRegGov when possible

Changes: https://git.openjdk.org/jdk/pull/10461/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10461&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8294261
  Stats: 103 lines in 6 files changed: 6 ins; 17 del; 80 mod
  Patch: https://git.openjdk.org/jdk/pull/10461.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10461/head:pull/10461

PR: https://git.openjdk.org/jdk/pull/10461