RFR: 8308076: X86_64: make rheapbase register allocatable in zero based compressedOops mode [v2]

[kuaiwei]

> In x86 64 mode, decode heap oop could use SIB without base if heap base is zero. like
> 
> 0d1     movl    R11, [,R9 << 3 + #72] (zero base compressed oop addressing) # compressed ptr ! Field: java/lang/ClassLoader.classAssertionStatus 
> 
> So rheapbase( r12 )  can be allocated as general register.
> 
> Tier 1/2 tests are passed without new failure.

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

  fix build error

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/13976/files
  - new: https://git.openjdk.org/jdk/pull/13976/files/bc7c539c..34ff1f61

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13976&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13976&range=00-01

  Stats: 10 lines in 4 files changed: 3 ins; 1 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/13976.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13976/head:pull/13976

PR: https://git.openjdk.org/jdk/pull/13976