[RFC][AARCH64] Machine reorg pass for aarch64/Falkor to handle prefetcher tag collision

[Kugan Vivekanandarajah]

Implements a machine reorg pass for aarch64/Falkor to handle
prefetcher tag collision. This is strictly not part of the loop
unroller but for Falkor, unrolling can make h/w prefetcher performing
badly if there are too much tag collisions based on the discussions in
https://gcc.gnu.org/ml/gcc/2017-10/msg00178.html.

gcc/ChangeLog:

2018-02-12  Kugan Vivekanandarajah  <kuganv@linaro.org>

    * config/aarch64/aarch64.c (iv_p): New.
    (strided_load_p): Likwise.
    (make_tag): Likesie.
    (get_load_info): Likewise.
    (aarch64_reorg): Likewise.
    (TARGET_MACHINE_DEPENDENT_REORG): Implement new target hook.
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