nir: fix deref comparison where a cast deref gets larger.

This might need more work, but it seems to fix the test I wrote.
10 jobs for !929 with nir-deref-compare-upsize-casts in 9 minutes and 32 seconds (queued for 3 seconds)
latest detached
Status Job ID Name Coverage
  Containers Build
passed #311608
debian

00:00:21

 
  Build+Test
passed #311610
meson-clang

00:05:00

passed #311614
meson-clover

00:08:23

passed #311613
meson-main

00:03:27

passed #311609
meson-swr-glvnd

00:05:04

passed #311615
meson-vulkan

00:02:00

passed #311617
scons-llvm

00:03:07

passed #311616
scons-nollvm

00:05:17

passed #311611
scons-swr

00:03:11

passed #311612
scons-win64

00:09:10