Enable string "array get" vectorization.

Rationale:
Like its scalar counterpart, the SIMD implementation of array get from
a string needs to deal with compressed and uncompressed cases.
Micro benchmarks shows 2x to 3x speedup for just copying data!

Test: test-art-target, test-art-host
Change-Id: I2fd714e50715b263123c215cd181f19194456d2b