[SelectionDAG] Legalize <1 x T> vector types for atomic load

[jofrn]

Scalarize vector of atomic load in SelectionDAG.

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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.