[PATCH] ARM NEON: Handle v16i8 and v8i16 reverse shuffles

[Arnold Schwaighofer]

Lower reverse shuffles to a vrev64 and a vext instruction instead of the default
legalization of storing and loading to the stack. This is important because we
generate reverse shuffles in the loop vectorizer when we reverse store to an
array.

  uint8_t Arr[N];
  for (i = 0; i < N; ++i)
    Arr[N - i - 1] = …

For v8i16 we now generate something like:

  vrev64.16       q9, q9
  vext.16 q9, q9, q9, #4

instead of:

  orr r1, r0, #14
  vst1.16 {d16[0]}, [r1, :16]
  orr r1, r0, #12
  vst1.16 {d16[1]}, [r1, :16]
  orr r1, r0, #10
  vst1.16 {d16[2]}, [r1, :16]
  orr r1, r0, #8
  vst1.16 {d16[3]}, [r1, :16]
  orr r1, r0, #6
  vst1.16 {d17[0]}, [r1, :16]
  orr r1, r0, #4
  vst1.16 {d17[1]}, [r1, :16]
  orr r1, r0, #2
  vst1.16 {d17[2]}, [r1, :16]
  vst1.16 {d17[3]}, [r0, :16]
  vld1.64 {d16, d17}, [r0, :128]


For v16i8 we now generate something like:

  vrev64.8        q8, q8
  vext.8  q8, q8, q8, #8

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