This can happen if assembling a reference to _GLOBAL_OFFSET_TABLE_.
While it doesn't make sense to try to assemble that for COFF,
the fact that we previously used llvm_unreachable meant that the code
had undefined behaviour if something tried to assemble that.
The configure script of libgmp would try to assemble such a snippet
(which should signal a failure). If llvm is built without assertions,
the undefined behaviour meant a (near) infinite loop.
Differential Revision: https://reviews.llvm.org/D52903
llvm-svn: 343811
27aec8037f