unittests: Make the expected value the first argument in EXPECT_EQ [NFC]

This improves the readability of failing test results, as gtest prints always
the first argument as the 'expected value'.

In the previous commit we already changed the tests for isl_valFromAPInt. In
this commit, the tests for IslValToAPInt follow.

Suggested-by: Michael Kruse <llvm@meinersbur.de>
llvm-svn: 279817