This transform is complementary to the `structured.pack` op which
allows packing a whole op but does not allow transposes on the individual
operands.
`structured.pack_transpose` allows transposing single operands connected to
pack or unpack ops after the fact.
This makes the system overall more composable than e.g. a giant transform
op with all permutation specified at once.
Differential Revision: https://reviews.llvm.org/D142053
790f237012