[LoongArch] Construct codegen infra and generate first add instruction.

This patch constructs codegen infra and successfully generate the first
'add' instruction. Add integer calling convention for fixed arguments which
are passed with general-purpose registers.

New test added here:

  CodeGen/LoongArch/ir-instruction/add.ll

The test file is placed in a subdirectory because we will use
subdirctories to distinguish different categories of tests (e.g.
 intrinsic, inline-asm ...)

Reviewed By: MaskRay, SixWeining

Differential Revision: https://reviews.llvm.org/D122366