[llvm-diff] Precommit: Add loop test case with forward reference

Diffing phi nodes was recently added to llvm-diff.
However, there currently is a limitation where equivalent values
cannot be detected as such, leading to false positive diff reports.

If a phi node refers a value defined in a basic block dominated by
the current basic block, for example a phi node in a loop header referring
a value defined in the loop body, we cannot prove equivalence of the referred
values, because the basic block containing the variable definition has
not yet been processed.

This commit adds a test case showing this behavior, serving as a precommit
for an upcoming fix of the above.

Differential Revision: https://reviews.llvm.org/D137262