[SelectionDAG] Make SelectCode return void

SelectCode has been returning nullptr since 182dac0 ("SDAG: Make
SelectCodeCommon return void", 2016-05-10). Make SelectCode also
return void instead, as all callers have been updated.

Patch by Sven van Haastregt.

Review: https://reviews.llvm.org/D30497
llvm-svn: 297377