[XLA:GPU] Clean up ir_emitter_triton_test.cc.
[XLA:GPU] Clean up ir_emitter_triton_test.cc.

This diff:
0. Replaces ASSERT with EXPECT in line with Google C++ style guide.
1. Expands single letter SSAs introduced by previous PR to make semantic meaning more verbose but also explicit.
2. Gets rid of non-pretty mixed checks of OK + booleans.
