RFR: 8295396: RISC-V: Cleanup useless CompressibleRegions

[Fei Yang]

On Mon, 17 Oct 2022 07:57:00 GMT, Xiaolin Zheng <xlinzheng at openjdk.org> wrote:

> Cleanup no longer used old code introduced in the riscv-port repo after #10643, in which we have marked out all incompressible places so all other generated instructions could be safely compressed if compressible. Hence the old `CompressibleRegion`s are useless and need a cleanup.
> 
> After this patch there are only two places using `CompressibleRegion`s: `MachNopNode::emit()` and `MacroAssembler::align()`. These `CompressibleRegion`s transforming the nops used for alignment purposes cannot be removed and the nops should be kept as 2-byte when RVC is enabled, for that we are at a 2-byte boundary and want to do `align(4)` with 4-byte nops would be impossible. So under RVC the nops for alignment purposes must be 2-byte ones. Others `CompressibleRegion`s are useless now.
> 
> Has passed hotspot tier1~4 together with other patches; another tier1 is running.

Looks good.

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

Marked as reviewed by fyang (Reviewer).

PR: https://git.openjdk.org/jdk/pull/10722