[mlir][scf] Relax type requirement on for

scf.for loop was restricted to only operate on Index type since
splitting out from affine.for. Relax requirement to allow for signless
integer types additionally. This allows specifying explicitly different
bitwidths for different loops as well as specialize from index to iN
while still using scf.for.

Differential Revision: https://reviews.llvm.org/D145288