Reduce default for -fmax-pmcheck-iterations from 1e7 to 2e6

The commit 28f951ed introduced the
`-fmax-pmcheck-iterations` flag and set the default limit to 1e7
iterations.

However, this value is still high enough that it can result GHC to
exhibit memory spikes beyond 1 GiB of RAM usage (heap profile showed
several `(:)`s, as well as `THUNK_2_0`, and `PmCon` during the memory
spikes)

A value of 2e6 seems to be a safer upper bound which still manages to
let the checker not run into the limit in most cases.

Test Plan: Validate, try building a few Hackage packages

Reviewers: austin, gkaracha, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2095