[AArch64][SVE] Support implicit lax vector conversions for SVE types

Lax vector conversions was behaving incorrectly for implicit casts
between scalable and fixed-length vector types. For example, this:

    #include <arm_sve.h>

    #define N __ARM_FEATURE_SVE_BITS
    #define FIXED_ATTR __attribute__((arm_sve_vector_bits(N)))

    typedef svfloat32_t fixed_float32_t FIXED_ATTR;

    void allowed_depending() {
      fixed_float32_t fs32;
      svfloat64_t s64;

      fs32 = s64;
    }

... would fail because the vectors have differing lane sizes. This patch
implements the correct behaviour for
-flax-vector-conversions={none,all,integer}. Specifically:

- -flax-vector-conversions=none prevents all lax vector conversions
  between scalable and fixed-sized vectors.
- -flax-vector-conversions=integer allows lax vector conversions between
  scalable and fixed-size vectors whose element types are integers.
- -flax-vector-conversions=all allows all lax vector conversions between
  scalable and fixed-size vectors (including those with floating point
  element types).

The implicit conversions are implemented as bitcasts.

Reviewed By: fpetrogalli

Differential Revision: https://reviews.llvm.org/D91067