RFR: 8336464: Force `CastX2P` to be a two-address instruction

[Fei Gao]

This patch forces `CastX2P` to be a two-address instruction, so that C2 could allocate the same register for `dst` and `src`. Then we can remove the instruction completely in the assembly.

The motivation comes from some cast operations like `castPP`. The difference for ADLC between `castPP` and `CastX2P` lies in that `CastX2P` always has different types for `dst` and `src`. We can force ADLC to generate an extra `two_adr()` for `CastX2P` like it does automatically for `castPP`, which could tell register allocator that the instruction needs the same register for `dst` and `src`.

However, sometimes, RA and GCM in C2 can't work as we expected.

For example, we have Assembly on the existing code:

  ldp    x10, x11, [x17,#136]
  add    x10, x10, x15
  add    x11, x11, x10
  ldr    x12, [x17,#152]
  str    x16, [x10]
  add    x10, x12, x15
  str    x16, [x11]
  str    x16, [x10]


After applying the patch independently, the assembly is:

  ldr    x10, [x16,#136]  <--- 1
  add    x10, x10, x15
  ldr    x11, [x16,#144]  <--- 2
  mov    x13, x10         <--- 3
  str    x17, [x13]
  ldr    x12, [x16,#152]
  add    x10, x11, x10
  str    x17, [x10]
  add    x10, x12, x15
  str    x17, [x10]


C2 generates a totally extra `mov`, see 3, and we even lost the chance to merge load pair, see 1 and 2. That's terrible.

Although this scenario would disappear after combining with https://github.com/openjdk/jdk/pull/20157, I'm still not sure if this patch is worthwhile.

-------------

Commit messages:
 - Use an interface function instead of macro
 - Force `CastX2P` to be a two-address instruction

Changes: https://git.openjdk.org/jdk/pull/20159/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20159&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8336464
  Stats: 65 lines in 9 files changed: 49 ins; 4 del; 12 mod
  Patch: https://git.openjdk.org/jdk/pull/20159.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20159/head:pull/20159

PR: https://git.openjdk.org/jdk/pull/20159