RFR: 8245756: Reduce bootstrap cost of StringConcatFactory prependers

[Claes Redestad]

Hi,

let's improve the StringConcatFactory a bit!

- prependers where both prefix and suffix is null are common in typical
expressions; caching these specifically is profitable

- current prepender argument ordering is unfortunate: String prefix,
<arg>, String suffix. This means we have to do two back-to-back
insertArguments rather than one if they were grouped together

- add a bridge to the String mix function in StringConcatHelper to avoid
method handle invokes in the bootstrap path.

Webrev: http://cr.openjdk.java.net/~redestad/8245756/open.00/
Bug:    https://bugs.openjdk.java.net/browse/JDK-8245756

All together I see a substantial improvement on a number of
concatenation bootstrap tests. On one stress test[1] I get these numbers
before:

      4,257,229,918 instructions # 1.10 insn per cycle ( +- 0.84% )
        847,230,764 branches # 504.826 M/sec ( +- 0.87% )
         34,600,787 branch-misses # 4.08% of all branches ( +- 0.91% )
        0.319545331 seconds time elapsed ( +- 1.18% )

And after:

      3,692,867,129 instructions # 1.10 insn per cycle ( +- 0.73% )
        734,054,515 branches # 498.753 M/sec ( +- 0.74% )
         30,699,692 branch-misses # 4.18% of all branches ( +- 0.75% )
        0.295842958 seconds time elapsed ( +- 1.10% )

So ~13% reduction on this test.

Testing: tier1+2

Thanks!

/Claes

[1] 
http://cr.openjdk.java.net/~redestad/scratch/erase_scf_types.00/ObjStringCombos.java