[mlir][OpenACC] Fix tile clause element bounds for descending loops (#227185)

acc.loops could have negative steps, but ACCLoopTiling computed each
element loop's bound assuming an ascending one. Both halves of that
computation were wrong for a descending dimension: the inclusive edge
subtracted one instead of adding one, and the bound was clamped with min
instead of max.

For `DO j = n, floor, -1` under `tile(32)`, the last tile's bound came
out as `min(floor, j - 33)`, which selects `j - 33` whenever it falls
below the floor. The element loop then ran past the end of the iteration
space, reading and writing outside the array.

With this change, we pick the clamp and the inclusive edge from the
step's direction. When the sign is only known at runtime, we compute
both and select on it.

The tiling tests had no coverage of negative steps or of inclusive upper
bounds. This patch adds four new tests: descending, descending with an
inclusive bound, ascending with an inclusive bound, and a runtime-signed
step.