ARM & AArch64: share the BI__builtin_neon enum defs.
ARM & AArch64: fully share NEON implementation of permutation intrinsics

As a starting point, this moves the CodeGen for NEON permutation
instructions (vtrn, vzip, vuzp) into a new shared function.
ARM & AArch64: extend shared NEON implementation to first block.

This extends the refactoring to the whole of the first block of
trivial correspondences (as a fairly arbitrary boundary).