Audit vector-transfer-flatten.mlir

[banach-space]

Audit vector-transfer-flatten.mlir (and the corresponding patterns) and add tests for scalable vectors.

Patches:

• [mlir][vector] Refactor vector-transfer-flatten.mlir (nfc) (3/n) #95745
• [mlir][vector] Refactor vector-transfer-flatten.mlir (nfc) (1/n) #95743
• [mlir][vector] Refactor vector-transfer-flatten.mlir (nfc) (2/n) #95744
• [mlir][vector] Add tests for xfer-flatten patterns #102210
• [mlir][vector][test] Split tests from vector-transfer-flatten.mlir #102584