RFR: 8221980: Simplify Optional implementation

[Stuart Marks]

On 4/4/19 3:29 AM, Claes Redestad wrote:
> the current code for Optional.ofNullable() null-checks the value, then
> calls of(), which calls new Optional(), which null-checks again via
> Objects.requireNonNull(). This can be simplified by refactoring so that we only 
> null-check once and chain through fewer methods.
> 
> Bug:    https://bugs.openjdk.java.net/browse/JDK-8221980
> Webrev: http://cr.openjdk.java.net/~redestad/8221980/open.00/
> 
> While JITs are likely to inline all this and generate the optimal
> code, it can fail to do so due inlining heuristics. This simplification
> reduces the risk of hitting such limits, while also speeding up code
> executed during startup and warmup phases.
> 
> Testing: tier1-2, verified a small improvement in startup profiles

Changes look good, thanks.

s'marks