[AlwaysInliner] Call mergeAttributesForInlining after inlining

Like inlineCallIfPossible and InlinerPass, after inlining mergeAttributesForInlining
should be called to merge callee's attributes to caller. But it is not called in
AlwaysInliner, causes caller's attributes inconsistent with inlined code.

Attached test case demonstrates that attribute "min-legal-vector-width"="512" is
not merged into caller without this patch, and it causes failure in SelectionDAG
when lowering the inlined AVX512 intrinsic.

Differential Revision: https://reviews.llvm.org/D91446