[mlir][llvm] Modernize LLVM instruction and global import (NFC).

Modernize the import of LLVMIR instructions and global variables.
Use longer variable names, factor out code used to import call or
invoke instructions, use the CPP builders for importing branch
instructions, etc. The revision is a preparation for a follow up
revision that moves the import code to implement improved error
handling.

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D139404