[llvm][GenericUniformity] Prevent assert while calculating temporal divergence

analyzeTemporalDivergence() was missing the check for always-uniform before
evaluating weather an instruction depends on a value defined in the cycle.
Fix for #60638
https://github.com/llvm/llvm-project/issues/60638

Reviewed By: sameerds, foad, #amdgpu

Differential Revision: https://reviews.llvm.org/D144070