[LoopVectorize][SVE] Fix crash when vectorising FP negation

This patch fixes a crash encountered when vectorising the following loop:

 void foo(float *dst, float *src, long long n) {
   for (long long i = 0; i < n; i++)
     dst[i] = -src[i];
 }

using scalable vectors. I've added a test to

 Transforms/LoopVectorize/AArch64/sve-basic-vec.ll

as well as cleaned up the other tests in the same file.

Differential Revision: https://reviews.llvm.org/D98054