[Verifier] Allow vector type in atomic load and store

[jofrn]

Vector types on atomics are assumed to be invalid by the verifier. However,
this type can be valid if it is lowered by codegen.

---

Stack:

• [X86] Add atomic vector tests for >1 sizes. #6
• [SelectionDAG][X86] Add floating point promotion. #5
• [SelectionDAG] Legalize <1 x T> vector types for atomic load #4
• [Verifier] Allow vector type in atomic load and store #3 ⬅

⚠️ Part of a stack created by spr. Do not merge manually using the UI - doing so may have unexpected results.