[ARM,CDE] Implement CDE vreinterpret intrinsics

[ARM,CDE] Implement CDE vreinterpret intrinsics

Summary:
This patch implements the following CDE intrinsics:

  int8x16_t __arm_vreinterpretq_s8_u8 (uint8x16_t in);
  uint16x8_t __arm_vreinterpretq_u16_u8 (uint8x16_t in);
  int16x8_t __arm_vreinterpretq_s16_u8 (uint8x16_t in);
  uint32x4_t __arm_vreinterpretq_u32_u8 (uint8x16_t in);
  int32x4_t __arm_vreinterpretq_s32_u8 (uint8x16_t in);
  uint64x2_t __arm_vreinterpretq_u64_u8 (uint8x16_t in);
  int64x2_t __arm_vreinterpretq_s64_u8 (uint8x16_t in);
  float16x8_t __arm_vreinterpretq_f16_u8 (uint8x16_t in);
  float32x4_t __arm_vreinterpretq_f32_u8 (uint8x16_t in);

These intrinsics are header-only because they reuse the existing
MVE vreinterpret clang built-ins.

This set is slightly different from the published specification
(see https://static.docs.arm.com/101028/0010/ACLE_2019Q4_release-0010.pdf):
it includes

  int8x16_t __arm_vreinterpretq_s8_u8 (uint8x16_t in);

which was unintentionally ommitted from the spec, and
does not include

  float64x2_t __arm_vreinterpretq_f64_u8 (uint8x16_t in);

The float64x2_t type requires additional implementation
effort, and we are not including it yet.

Reviewers: simon_tatham, MarkMurrayARM, dmgreen, ostannard

Reviewed By: MarkMurrayARM

Subscribers: kristof.beyls, danielkiss, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D76300