[AArch64][SVE] Asm: Support for vector element compares.

This patch adds instructions for comparing elements from two vectors, e.g.
  cmpgt p0.s, p0/z, z0.s, z1.s

and also adds support for comparing to a 64-bit wide element vector, e.g.
  cmpgt p0.s, p0/z, z0.s, z1.d

The patch also contains aliases for certain comparisons, e.g.:
  cmple p0.s, p0/z, z0.s, z1.s => cmpge p0.s, p0/z, z1.s, z0.s
  cmplo p0.s, p0/z, z0.s, z1.s => cmphi p0.s, p0/z, z1.s, z0.s
  cmpls p0.s, p0/z, z0.s, z1.s => cmphs p0.s, p0/z, z1.s, z0.s
  cmplt p0.s, p0/z, z0.s, z1.s => cmpgt p0.s, p0/z, z1.s, z0.s

llvm-svn: 334931