Order CALLSEQ_START and CALLSEQ_END nodes.

Fixes PR16146: gdb.base__call-ar-st.exp fails after
pre-RA-sched=source fixes.

Patch by Xiaoyi Guo!

This also fixes an unsupported dbg.value test case. Codegen was
previously incorrect but the test was passing by luck.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@182885 91177308-0d34-0410-b5e6-96231b3b80d8