[llgo] Elide alloca for unused received values in select
Summary: If a receive case in a select statement is not assigned to a named variable, then we can eliminate the alloca and copy at runtime.
Test Plan: lit test added
Reviewers: pcc
Reviewed By: pcc
Subscribers: llvm-commits
Differential Revision: http://reviews.llvm.org/D6785
llvm-svn: 225033
