Make VariableOp instructions be in the first block in the function.

Since VariableOp is serialized during processBlock, we add two more fields,
`functionHeader` and `functionBody`, to collect instructions for a function.
After all the blocks have been processed, we append them to the `functions`.

Also, fix a bug in processGlobalVariableOp. The global variables should be
encoded into `typesGlobalValues`.

PiperOrigin-RevId: 280105366