[Attributor] Add an option to increase the max number of iterations

Right now the Attributor defaults to 32 fixed point iterations unless it is set
explicitly by a command line flag. This patch allows this to be configured when
the attributor instance is created. The maximum is then increased in OpenMPOpt
if the target is a kernel. This is because the globalization analysis can result
in larger iteration counts due to many dependent instances running at once.

Depends on D102444

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D104416