[mlir][LLVMIR] "Modernize" Insert/ExtractValueOp

This patch "modernizes" the LLVM `insertvalue` and `extractvalue`
operations to use DenseI64ArrayAttr, since they only require an array of
indices and previously there was confusion about whether to use i32 or
i64 arrays, and to use assembly format.

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D131537