[mlir] Translate global initializers after creating all LLVM IR globals

In case an operation in a global initializer region refers to another
global variable defined afterwards in the module of itself, translation
to LLVM IR was currently crashing because it could not find the LLVM IR global
when going through the initializer block.

To solve this problem, split global conversion to LLVM IR into two passes. A
first pass that creates LLVM IR global variables, and a second one that converts
the initializer, if any, and adds it to the llvm global.

Differential Revision: https://reviews.llvm.org/D99246