Index analysis of while dynamic slice

If we modify a specific index of a tensor and all reads to that tensor are performed prior to the write we can replace the slice operand with the value outside the loop

The issue describes index analysis for a while loop with a dynamic slice, but names no file, test, or entry point. Locate the relevant analysis in Enzyme-JAX and establish a reproducer for the stated read-before-write case. Done means the slice operand can be replaced outside the loop when the stated conditions hold.

Written by the indexing model from the issue text.