[MLIR][Arith] Fix arith::AtomicRMWKind::maximumf's identity value (#70312)

-- In order to compute maximum, we should always initialise the
   result with the largest negative value possible for the concerned
   element type, instead of the smallest.
-- This commit essentially adds a fix to this issue.

Signed-off-by: Abhishek Varma <abhishek@nod-labs.com>