ConvertToCiphertextSemantics: support dynamic cleartext indices for tensor.extract kernel

[j2kun]

(neglected to add a TODO)

This is in the conversion pattern for ConvertTensorExtractNewLayout (which may be renamed to ConvertTensorExtract after #2199).

[github-actions[bot]]

This issue has 2 outstanding TODOs:

• lib/Transforms/ConvertToCiphertextSemantics/ConvertToCiphertextSemantics.cpp:964: Support dynamic indices in tensor.extract
• tests/Transforms/convert_to_ciphertext_semantics/BUILD:10: Support dynamic indices

This comment was autogenerated by todo-backlinks