[mlir] Add Repeated<T> constructors for TypeRange and ValueRange (#186923)

Many MLIR APIs end up using a range of the same Type / Value repeated N
times, due to the (function of the) dimensionality of the problem.
Allocating a vector of N identical element is wasteful.

Add `Repeated<T>` as PointerUnion variants in TypeRange and ValueRange,
enabling O(1) storage for repeated elements. Size remains 2 pointers (16
bytes on 64-bit) for both range types. This required variable-width
`PointerUnion` encoding added in
https://github.com/llvm/llvm-project/pull/188167 on 32-bit systems.

Also update several MLIR dialects and conversions to exercise the new
code.

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Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
GitOrigin-RevId: 107d8539d0451c8b079814d23498aa23b2cde1f5