AArch64/ARM64: add more specific diagnostic for invalid vector lanes
AArch64/ARM64: add more specific diagnostic for floating imm 0.0.
AArch64/ARM64: make NEON vector list parsing a bit more robust

It doesn't change the results, but it seems silly not to diagnose obvious
problems early on.
AArch64/ARM64: enable MC-level diagnostic tests for NEON insts.

Obviously we can't expect the two backends to produce identical diagnostics,
since what's possible depends quite a bit on how the .td files are structured.
I think the ARM64 diagnostics are basically of the same quality in all the
changed cases, so I've split the CHECK lines.