Changed HashMap.getOrElseUpdate to only calculate the index once
Changed HashMap.getOrElseUpdate to only calculate the index once

Fixes https://issues.scala-lang.org/browse/SI-10049

Since `groupBy` uses this method extensively and suffered a measurable slowdown in `2.12.0`, this modification restores (and exceeds) its original speed.

---
included benchmarks:

(`ns/op` → smaller is better)

`before (2.12.0):`
```java
Benchmark                                     (size)  Mode  Cnt      Score      Error  Units
s.c.immutable.VectorMapBenchmark.groupBy          10  avgt   20    865.693 ±    7.869  ns/op
s.c.immutable.VectorMapBenchmark.groupBy         100  avgt   20   3095.657 ±   56.438  ns/op
s.c.immutable.VectorMapBenchmark.groupBy        1000  avgt   20  28247.005 ±  470.513  ns/op
s.c.mutable.HashMapBenchmark.get                  10  avgt   20    679.448 ±   11.809  ns/op
s.c.mutable.HashMapBenchmark.get                 100  avgt   20   7240.178 ±   61.734  ns/op
s.c.mutable.HashMapBenchmark.get                1000  avgt   20  95725.127 ± 2373.458  ns/op
s.c.mutable.HashMapBenchmark.getOrElseUpdate      10  avgt   20    836.561 ±   20.085  ns/op
s.c.mutable.HashMapBenchmark.getOrElseUpdate     100  avgt   20   7891.368 ±   56.808  ns/op
s.c.mutable.HashMapBenchmark.getOrElseUpdate    1000  avgt   20  97478.629 ± 1782.497  ns/op
s.c.mutable.HashMapBenchmark.put                  10  avgt   20    243.422 ±    2.915  ns/op
s.c.mutable.HashMapBenchmark.put                 100  avgt   20   5810.927 ±   60.054  ns/op
s.c.mutable.HashMapBenchmark.put                1000  avgt   20  82175.539 ± 1690.296  ns/op
```
`after:`
```java
Benchmark                                     (size)  Mode  Cnt      Score      Error  Units
s.c.immutable.VectorMapBenchmark.groupBy          10  avgt   20    627.007 ±    9.718  ns/op
s.c.immutable.VectorMapBenchmark.groupBy         100  avgt   20   2086.955 ±   19.042  ns/op
s.c.immutable.VectorMapBenchmark.groupBy        1000  avgt   20  19515.234 ±  173.647  ns/op
s.c.mutable.HashMapBenchmark.get                  10  avgt   20    683.977 ±   11.843  ns/op
s.c.mutable.HashMapBenchmark.get                 100  avgt   20   7345.675 ±   41.092  ns/op
s.c.mutable.HashMapBenchmark.get                1000  avgt   20  95085.926 ± 1702.997  ns/op
s.c.mutable.HashMapBenchmark.getOrElseUpdate      10  avgt   20    503.208 ±    2.643  ns/op
s.c.mutable.HashMapBenchmark.getOrElseUpdate     100  avgt   20   5526.483 ±   28.262  ns/op
s.c.mutable.HashMapBenchmark.getOrElseUpdate    1000  avgt   20  69265.900 ±  674.958  ns/op
s.c.mutable.HashMapBenchmark.put                  10  avgt   20    252.481 ±    7.597  ns/op
s.c.mutable.HashMapBenchmark.put                 100  avgt   20   5708.034 ±  110.360  ns/op
s.c.mutable.HashMapBenchmark.put                1000  avgt   20  82051.378 ± 1432.009  ns/op
```

i.e. for the given benchmark conditions `~40%` faster `groupBy` and `getOrElseUpdate`

lrytz/scala,shimib/scala,lrytz/scala,shimib/scala,scala/scala,scala/scala,scala/scala,scala/scala,martijnhoekstra/scala,lrytz/scala,shimib/scala,felixmulder/scala,felixmulder/scala,scala/scala,jvican/scala,felixmulder/scala,martijnhoekstra/scala,felixmulder/scala,jvican/scala,martijnhoekstra/scala,felixmulder/scala,lrytz/scala,shimib/scala,shimib/scala,jvican/scala,jvican/scala,martijnhoekstra/scala,shimib/scala,felixmulder/scala,lrytz/scala,jvican/scala,scala/scala,jvican/scala,martijnhoekstra/scala,felixmulder/scala,martijnhoekstra/scala,jvican/scala,lrytz/scala