StringConcat MH_INLINE_SIZED_EXACT should skip storage initialization

MH_INLINE_SIZED_EXACT strategy is known to win in many cases where OptimizeStringConcat does not work, but it is penalized when OptimizeStringConcat does work:
 http://cr.openjdk.java.net/~shade/8150463/perf-bad.txt

"perfasm" profiling shows this is because MH_INLINE_SIZED_EXACT has to allocate the underlying storage from Java, which requires zeroing; while C2 is able to avoid zeroing altogether, because it knows the array would get overwritten completely:
  http://cr.openjdk.java.net/~shade/8150463/BC_SB.perfasm
  http://cr.openjdk.java.net/~shade/8150463/MH_INLINE_SIZED_EXACT.perfasm

We need to solve this before switching to most optimal concat strategy.