[Thumb2][MVE] Recognise shuffle truncation patterns suitable for ARMISD::MVETRUNC

[Thumb2][MVE] Recognise shuffle truncation patterns suitable for ARMISD::MVETRUNC

I'm helping with the remaining regressions on D127115, and one of my candidate fixes caused some regressions with MVE interleaved shuffles due to poor handling of 'truncation' style shuffle masks (0,2,4,6,...).

This patch attempts to use the ARMISD::MVETRUNC node to handle these cases, based off existing code in LowerTruncate.

It handles both (0,2,4,6,...) and (1,3,5,7,....) 'top' style patterns (assuming no endian problems). I shift down the 'top' patterns - a basic search of ARM docs suggests MVE has some top/bottom truncation/narrowing instructions but I don't seem to be able to get them to be used.

Differential Revision: https://reviews.llvm.org/D141791