RFR: 8351949: RISC-V: Cleanup and enable store-load peephole for membars

[Robbin Ehn]

Hi please consider.

|RVWMO| Patched|
| ---------- | ---------- |
|fence iorw,iorw| fence iorw,ow|
|sw  t4,120(t2) | sw  t4,120(t2) |
|fence ow,ir | unnecessary_membar_volatile_rvwmo |
| sw  t6,128(t2) // Non-volatile | sw  t6,128(t2) // Non-volatile |
|fence iorw,ow | fence iorw,ow|
|sw  t5,124(t2) |sw  t5,124(t2) |

|TSO                                | Patched|
| ---------- | ---------- |
| lw	a4,120(t2) | lw	a6,120(t2) |
| sw	a0,124(t2) | sw	t6,124(t2) |
| fence	iorw,iorw | unnecessary_membar_volatile_tso |
| sw	t4,120(t2) | sw	t4,120(t2) |
| fence	ow,ir | unnecessary_membar_volatile_tso |
| sw	t6,128(t2) | sw	t5,128(t2) |
| sw	t5,124(t2) // Non-volatile| sw	a1,124(t2)  // Non-volatile |
| fence	iorw,iorw  | unnecessary_membar_volatile_tso |
|...  | ... |
| sw	a3,120(t2) | sw	a0,120(t2) |
| fence	ow,ir | fence	ow,ir |
| lw	a7,124(t2) | lw	a5,124(t2) |

For the specific rvwmo volatile store + store + volatile store is around 30% faster on VF2.

The patch do:
- Separate ztso and rvwmo in ad by using UseZtso predicate.
- Match all that requires the same membar.
- Make fence/fencei protected as they shouldn't be using directly.
- Increased cost of membars to VOLATILE_REF_COST.
- Added a real_empty pipe.
- Change to pipe_slow on TSO (as x86).

Note that C2-rv64 is now superior to gcc/clang regrading fencing:
https://godbolt.org/z/6E3YTP15j

Testing jcstress, tier1 and manually reading the generated assembly.
Doing additional testing, but RFR it now as it may need some consideration.

/Robbin

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

Commit messages:
 - Fixed ws
 - Revert NC
 - Fixed comment
 - UseNewCode

Changes: https://git.openjdk.org/jdk/pull/24035/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24035&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8351949
  Stats: 148 lines in 4 files changed: 72 ins; 27 del; 49 mod
  Patch: https://git.openjdk.org/jdk/pull/24035.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24035/head:pull/24035

PR: https://git.openjdk.org/jdk/pull/24035