AArch64/ARM64: produce more informative diagnostic assembling some immediates

No tests here, they'll be added when the entire neon-diagnostics.s test from
AArch64 is enabled.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208079 91177308-0d34-0410-b5e6-96231b3b80d8