[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