[XLA] Fix dynamic expression edge cases and batch bucketing

[stevenvar]

Summary

• treat unknown dynamic expressions as neither constants nor valid dynamic expressions, and avoid emitting them as the integer value -1
• preserve division expressions with a zero divisor instead of simplifying {0 / 0} to {1}
• keep configured XLA batch buckets immutable so batch selection is deterministic and safe for concurrent compilation requests
• remove stale debug-only output and clarify the StridedSlice expression fallback

Tests

• //tensorflow/compiler/jit:xla_batch_matcher_test checks request-order independence and immutable configured buckets
• @local_xla//xla:shape_test checks unknown-expression classification and zero-divisor simplification
• @local_xla//xla/service/llvm_ir:llvm_util_test checks that unknown expressions are not emitted as constants

All three focused test targets pass without cached test results.