[nnc][tests] Convert a bunch of FileCheck to checkIR
Summary:
I added a helper to convert a Stmt to string and FileCheck it, so
started using it in a bunch of places. I replaced about half the current uses,
got tired, started to write a Perl script to automate it, realized that was
hard, and decided to give up for a bit. But this cleans up some of the tests a
bit, so seems easy to review and worth landing.
Test Plan: test_tensorexpr --gtest_filter=LoopNest.*
Reviewed By: navahgar
Differential Revision: D27375866
fbshipit-source-id: 15894b9089dec5cf25f340fe17e6e54546a64257
