[mlir][llvm] Use longer variable names in LLVM IR import (NFC).

Rename single letter member variables and function arguments to use
longer names in ConvertFromLLVMIR.cpp. Also drop some uses of auto in
favor our spelling out the type and refactor some llvm::enumerate loops.

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D136246