[ACC] Use ExistingOps strictness in ACCSpecializeForDevice for non-specialized functions (#187645)

For non-specialized functions, ACCSpecializeForDevice collects ACC ops
inside compute constructs and applies device specialization patterns via
applyOpPatternsGreedily. With the default AnyOp strictness, the greedy
driver expands the worklist to parent ops when inner ops are modified,
accidentally unwrapping the parent acc.parallel via
ACCRegionUnwrapConversion. This leaves orphaned acc.loop
combined(parallel) ops that lose their parallelism and reduction
information downstream.

Set GreedyRewriteStrictness::ExistingOps so the greedy driver only
processes the initially collected inner ops, preserving the parent
compute construct for ACCComputeLowering to handle.

GitOrigin-RevId: 20768a95765468ab64c6dd5674ce8f4e83c0a742