[BugFix][Hopper] Select the widest legal WGMMA N instead of gcd (#2931)

* [BugFix][Hopper] Select the widest legal WGMMA N instead of gcd

block_N extents that are legal single WGMMA widths but not powers of two
(96, 112, 160, 176) were decomposed into n32 / n16 instructions, because
the instruction width came from gcd(warp_col_tiles, 256). wgmma.h already
instantiates every multiple of 8 up to 256, so only the width selection
was the constraint.

Select the widest legal width that tiles warp_col_tiles exactly, keeping
gcd as the fallback. Widening is restricted to multiples of 16: the
N % 16 == 8 class already miscomputes (#2593), and the integer instruction
tables only instantiate multiples of 16 (plus 8 and 24). The result is
never narrower than before, so no existing shape regresses.

The sparse generator carried the same line and now shares the helper.

* [Testing] Cover T.gemm_sp at the widened WGMMA N values

The width selector is shared with the sparse generator, so assert the emitted
wgmma_sp instruction and the sparse numeric result for block_N in {96, 112,
160} as well. Verified as a real regression: with the selector reverted the
sparse path emits n32 / n16 / n32 exactly like the dense one.