[GlobalISel] Introduce InlineAsmLowering class

Summary:
Similar to the CallLowering class used for lowering LLVM IR calls to MIR calls,
we introduce a separate class for lowering LLVM IR inline asm to MIR INLINEASM.

There is no functional change yet, all existing tests should pass.

Reviewers: arsenm, dsanders, aemerson, volkan, t.p.northover, paquette

Reviewed By: aemerson

Subscribers: gargaroff, wdng, mgorny, rovka, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D78316