[llvm] Support invariant.load on readonly intrinsics (#205916)

Update passes and analyses that look at invariant.load (ex. MemorySSA)
to also consider the possibility that `!invariant.load` may be present
on intrinsic calls.

Updates the verifier to indicate that `!invariant.load` is only legal
for loads and readonly intrinsics (and moves the definition of the
metadata out of the definition of `load`).

This has not been extended to arbitrary function calls since it's less
clear what the semantics of !invariant.load on them would be.
Furthermore, extension to read/write intrinsics like memcpy() is planned
as a followup.

This PR also deletes tests that were pre-committed in
https://github.com/llvm/llvm-project/pull/205894 but that become invalid
with the verifier changes in this PR

AI disclosure: I've reviewed the code but all the actual generation was
done by AI.

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Co-authored-by: Codex <codex@openai.com>