[mlir][OpenMP] Add optional alloc region to reduction decl (#102522)

This region is intended to separate alloca operations from reduction
variable initialization. This makes it easier to hoist allocas to the
entry block before control flow and complex code for initialization.

The verifier checks that there is at most one block in the alloc region.
This is not sufficient to avoid control flow in general MLIR, but by the
time we are converting to LLVMIR structured control flow should already
have been lowered to the cf dialect.

1/3
Part 2: https://github.com/llvm/llvm-project/pull/102524
Part 3: https://github.com/llvm/llvm-project/pull/102525

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