[mlir][llvm] Call the instruction builders from a static method (NFC).

Extract a static method to call the MLIR builders that translate LLVM IR
instructions to MLIR LLVM dialect operations. This change ensures the
MLIR builders have to use the moduleImport argument rather than calling
the import methods directly. As a result, both the intrinsic and the
instruction MLIR builders have to use a moduleImport argument and none
of them has direct access to private moduleImport methods. The revision
thus enforces consistent MLIR builder implementations for instructions
and intrinsics.

The revision also moves parseDataLayoutAlignment closer to its use.

Reviewed By: Dinistro

Differential Revision: https://reviews.llvm.org/D141265