RFR (S) 8150102: C1 should fold arraylength for constant/trusted arrays

[Aleksey Shipilev]

Hi,

Please review a small tuneup in C1 arraylength folding:
  https://bugs.openjdk.java.net/browse/JDK-8150102
  http://cr.openjdk.java.net/~shade/8150102/webrev.00

After this patch, C1 successfully folds the general arraylength
accesses. In the end, this lets to fold String.length() for constant
Strings:
  http://cr.openjdk.java.net/~shade/8150102/FoldArrayLength.java

I am actually wondering if as_LoadField path is ever hit in
applications, because the targeted microbenchmark for static final array
had improved with new as_Constant path.

Testing: JPRT -testset hotspot (half of the platforms completed, now
infra is stalled), microbenchmarks

Cheers,
-Aleksey

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