CodeGen: Check for a terminator in llvm::getFuncletMembership

Check for an end iterator from MachineBasicBlock::getFirstTerminator in
llvm::getFuncletMembership.  If this is turned into an assertion, it
fires in 48 X86 testcases (for example,
CodeGen/X86/regalloc-spill-at-ehpad.ll).

Since this is likely a latent bug (shouldn't all basic blocks end with a
terminator?) I've filed PR28938.

llvm-svn: 278344