RFR: 8277417: C1 LIR instruction for load-klass [v2]

[Roman Kennke]

> In C1, the load of a Klass* out of an object is currently identified by a load of type T_ADDRESS with offset oopDest::klass_offset_in_bytes(). When encountering such load, this may be decoded when +CompressedClassPointers. This is problematic and ugly: if we ever emit a T_ADDRESS load with offset 8 or 4 (== klass_offset_in_bytes) that is not a Klass*, we would attempt to decode the result. We have been lucky so far.
> 
> Also, in Lilliput, we want to do something entirely different there, and need to be able to emit more complex code, possibly including runtime call.
> 
> The change introduces a new C1 LIR opcode OpLoadKlass, and refactors the implementations in c1_LIRAssembler_xyz.cpp to emit the code there, instead of mem2reg(). Notice that I could not test anything but x86, all other platforms only received very basic testing via GHA. It would be nice if respective maintainers could give it a try.
> 
> Testing:
>  - [x] tier1 (x86_64)
>  - [x] tier2 (x86_64)
>  - [x] tier3 (x86_64)

Roman Kennke has updated the pull request incrementally with one additional commit since the last revision:

  Fix null-check on PPC

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/6464/files
  - new: https://git.openjdk.java.net/jdk/pull/6464/files/988036fa..3454c1bf

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=6464&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=6464&range=00-01

  Stats: 7 lines in 1 file changed: 6 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6464.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6464/head:pull/6464

PR: https://git.openjdk.java.net/jdk/pull/6464