Fix alloca_instruments_all_paddings.cc test to work under higher -O levels (llvm part)

When AddressSanitizer only a single dynamic alloca and no static allocas, due to an early exit from FunctionStackPoisoner::poisonStack we forget to unpoison the dynamic alloca.  This patch fixes that.

Reviewed at http://reviews.llvm.org/D7810

llvm-svn: 230316