[GC] improve testing around gc.relocate and fix a test

[GC] improve testing around gc.relocate and fix a test

Patch by: Ramkumar Ramachandra <artagnon@gmail.com>

"This patch started out as an exploration of gc.relocate, and an attempt
to write a simple test in call-lowering. I then noticed that the
arguments of gc.relocate were not checked fully, so I went in and fixed
a few things. Finally, the most important outcome of this patch is that
my new error handling code caught a bug in a callsite in
stackmap-format."

Differential Revision: http://reviews.llvm.org/D6824

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@225412 91177308-0d34-0410-b5e6-96231b3b80d8