ARM NEON relax parse time diagnostics for alignment specifiers.

ARM NEON relax parse time diagnostics for alignment specifiers.

There's more variation that we need to handle. Error checking will need
to be on operand predicates.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@146884 91177308-0d34-0410-b5e6-96231b3b80d8