[AArch64][GlobalISel] Support the 'returned' parameter attribute.

On AArch64 (which seems to be the only target that supports it), this
attribute allows codegen to avoid saving/restoring the value in x0
across a call.

Gives a 0.1% geomean -Os code size improvement on CTMark.

Differential Revision: https://reviews.llvm.org/D96099