RFR(S): 8155612: Aarch64: vector nodes need to support misaligned offset

[Roland Westrelin]

http://cr.openjdk.java.net/~roland/8155612/webrev.00/

When the superword optimization encounters a misaligned offset, it tries
to align the loop induction variable so the memory access is aligned. So
even though the aarch64 port only supports align vector memory accesses
( Matcher::misaligned_vectors_ok() return false), vector node can still
get a misaligned offset which results in:

# Internal Error
(/scratch/rwestrel/hs-comp/hotspot/src/cpu/aarch64/vm/assembler_aarch64.hpp:251),
pid=3780, tid=3807
# guarantee(chk == -1 || chk == 0) failed: Field too big for insn

Roland.