Don't try to spill static allocas when emitting expr cleanups with branches

Credit goes to Gor Nishanov for putting together the fix in
https://reviews.llvm.org/D33733!

This patch is essentially me patching it locally and writing some test
cases to convince myself that it was necessary for GNU statement
expressions with branches as well as coroutines. I'll ask Gor to land
his patch with just the coroutines test.

During LValue expression evaluation, references can be bound to
anything, really: call results, aggregate temporaries, local variables,
global variables, or indirect arguments. We really only want to spill
instructions that were emitted as part of expression evaluation, and
static allocas are not that.

llvm-svn: 304335