NEON: properly implement _high intrinsics

[easyaspi314]

High intrinsics merely have an implicit vget_high or vcombine as a helper for most of the widen or narrow instructions since 64-bit can't address the upper halves of registers anymore. There is no need to complicate them further.

[mr-c]

@easyaspi314 hey-o, does this PR need more work or should I rebase and merge?