This patch adds `!nonnull/!align` for load from reference to improve
codegen.

Generate nuw GEPs for struct member accesses, as inbounds + non-negative
implies nuw.

Regression tests are updated using update scripts where possible, and by
find + replace where not.

This makes codegen for array initialization simpler in two ways:
1. Drop the zero-index GEP at the start, which is no longer needed with
opaque pointers.
2. Emit GEPs directly to the correct element, instead of having a long
chain of +1 GEPs. This is more canonical, and also avoids regressions in
unoptimized builds from #93823.

Fix #69214 
In `emitOMPSimdRegion`, the `EmitOMPPrivateLoopCounters` should be after
`EmitOMPPrivateClause`.
If not, the private variables will be registered too early, which is not
allowed by `EmitOMPPrivateClause`.