RFR: 8259778: Merge MutableSpace and ImmutableSpace

[Serguei Spitsyn]

On Wed, 27 Jan 2021 23:06:41 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

> Please review this change which merges ImmutableSpace into MutableSpace,
> eliminating the former.  There were no interesting uses of ImmutableSpace,
> other than as the base class for MutableSpace.  The name ImmutableSpace is
> kind of a misnomer given that usage.
> 
> Testing:
> mach5 tier1-3

LGTM

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Marked as reviewed by sspitsyn (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/2271