[AMDGPU] improve fragile test for divergent branches

Summary:
The affected LIT test intends to test the correct use of divergence
analysis to detect a divergent branch with a uniform predicate. The
passes involved are LLVM IR passes, but the test runs llc and tries to
match against generated ISA, which makes it hard to demonstrate that
the intended behavior was really tested. Replaced this with a test
that invokes opt on the required passes and then checks for the
appropriate changes in the LLVM IR.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D75267