[mlir][llvm] Introduce a mapValue function in LLVMIR import (nfc).

The revision adds a mapValue function to the Importer, which can be used
in the MLIR builders to provide controlled accesses to the result
mapping of the imported instructions. Additionally, the change allows us
to avoid accessing a private member variable of the Importer class,
which simplifies future refactorings that aim at factoring out a
conversion interface (similar to the MLIR to LLVM translation). The
revision also renames the variables used when emitting the MLIR builders
to prepare the generalization to non-intrinsic instructions. In
particular, it renames callInst to inst and it passes in the instruction
arguments using an llvmOperands array rather than accessing the call
arguments directly.

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D135645