[AArch64][SVE] Asm: Support for SPLICE instruction.

The SPLICE instruction splices two vectors into one vector using a
predicate. It copies the active elements from the first vector, and
then fills the remaining elements with the low-numbered elements from
the second vector.

The instruction has the following form, e.g.

  splice z0.b, p0, z0.b, z1.b

for 8-bit elements. It also supports 16, 32 and
64-bit elements.

llvm-svn: 337253