[mlir] Squash LLVM_ArmNeon dialect into ArmNeon

The two dialects are largely redundant. The former was introduced as a mirror
of the latter operating on LLVM dialect types. This is no longer necessary
since the LLVM dialect operates on built-in types. Combine the two dialects.

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D98060