vtn: fix load/store to use single load/stores

The casting via a vector leading to alignment issues, as vload
only have to be element aligned not vector aligned.

There probably needs to be something that vectorises these load
/stores later if required.
10 jobs for !953 with vtn-nir-add-shuffle-fix-loadstore in 60 minutes and 44 seconds (queued for 7 minutes and 46 seconds)
latest detached
Status Job ID Name Coverage
  Containers Build
passed #319732
debian

00:00:25

 
  Build+Test
passed #319734
meson-clang

00:10:15

passed #319738
meson-clover

00:10:20

passed #319737
meson-main

00:05:12

passed #319733
meson-swr-glvnd

00:04:51

passed #319739
meson-vulkan

00:03:43

passed #319741
scons-llvm

00:06:04

passed #319740
scons-nollvm

00:04:25

passed #319735
scons-swr

00:08:07

passed #319736
scons-win64

00:07:18