[ARM] Implement division using vrecpe, vrecps

[Prathamesh Kulkarni <prathamesh dot kulkarni at linaro dot org>]

Hi,
This is a rebased version of patch that adds a pattern to neon.md for
implementing division with multiplication by reciprocal using
vrecpe/vrecps with -funsafe-math-optimizations excluding -Os.
The newly added test-cases are not vectorized on armeb target with
-O2. I posted the analysis for that here:
https://gcc.gnu.org/ml/gcc-patches/2016-05/msg01765.html

Briefly, the difference between little and big-endian vectorizer is in
arm_builtin_support_vector_misalignment() which calls
default_builtin_support_vector_misalignment() for big-endian case, and
that returns false because
movmisalign_optab does not exist for V2SF mode. This isn't observed
with -O3 because loop peeling for alignment gets enabled.

It seems that the test cases in patch appear unsupported on armeb,
after r221677 thus this patch requires no changes to
target-supports.exp to adjust for armeb (unlike last time which
stalled the patch).

Bootstrap+tested on arm-linux-gnueabihf.
Cross-tested on arm*-*-* variants.
OK for trunk ?

Thanks,
Prathamesh

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