After merging #159206 the new tests added would fail when verifying
machine code instructions with:
```
*** Bad machine code: Non-terminator instruction after the first terminator ***
- function: has_funclet
- basic block: %bb.4 call.block.4 (0x8000f837e8)
- instruction: SEH_SplitChained
First terminator was: RET64 $eax
*** Bad machine code: Non-terminator instruction after the first terminator ***
- function: has_funclet
- basic block: %bb.4 call.block.4 (0x8000f837e8)
- instruction: SEH_EndPrologue
First terminator was: RET64 $eax
```
This change removes `SEH_SplitChained` and replaces it with
`SEH_SplitChainedAtEndOfBlock` which acts as a marker to `X86AsmPrinter`
to emit the `.seh_splitchained` directive (and `.seh_endprolog`) when
the current block has finished.
f3aa84c810