[ScopInfo] Simplify domains early

This speeds up scop modeling for scops with many redundent existentially
quantified constraints. For the attached test case, this change reduces
scop modeling time from minutes (hours?) to 0.15 seconds.

This change resolves a compilation timeout on the AOSP build.

Thanks Eli for reporting _and_ reducing the test case!

Reported-by: Eli Friedman <efriedma@codeaurora.org>
llvm-svn: 303600