[AArch64][SVE] Allow lax conversion between VLATs and GNU vectors

Previously, lax conversions were only allowed between SVE vector-length
agnostic types and vector-length specific types. This meant that code
such as the following:

    #include <arm_sve.h>
    #define N __ARM_FEATURE_SVE_BITS
    #define FIXED_ATTR __attribute__ ((vector_size (N/8)))
    typedef float fixed_float32_t FIXED_ATTR;

    void foo() {
        fixed_float32_t fs32;
        svfloat64_t s64;
        fs32 = s64;
    }

was not allowed.

This patch makes a minor change to areLaxCompatibleSveTypes to allow for
lax conversions to be performed between SVE vector-length agnostic types
and GNU vectors.

Differential Revision: https://reviews.llvm.org/D91696