RFR: 8296170: Refactor stack-locking path in C2_MacroAssembler::fast_unlock() [v3]

[Roman Kennke]

> The code in C2_MacroAssembler::fast_unlock() has several (minor) issues: 
>  - The stack-locking path for x86_32 is not under UseHeavyMonitors - it would be executed even when stack-locking is disabled. 
>  - The stack-locking paths are the same for x86_32 and x86_64 - they can be merged into a common path. 
>  - In x86_32 path, we call get_thread(boxReg) which is totally bogus because we clear boxReg right afterwards with xorptr(boxReg, boxReg). 
>  - In x86_32 path, the CheckSucc label is identical to the DONE label, and in-fact CheckSucc is only ever really used in the x86_64 path and can be moved there.
> 
> Testing:
>  - [x] tier1 (x86_32, x86_64)
>  - [x] tier2 (x86_32, x86_64)

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

  Remove comments about DONE_LABEL being a hot target

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/10936/files
  - new: https://git.openjdk.org/jdk/pull/10936/files/bc4fd918..153353ac

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10936&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10936&range=01-02

  Stats: 12 lines in 1 file changed: 0 ins; 12 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/10936.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10936/head:pull/10936

PR: https://git.openjdk.org/jdk/pull/10936