[SystemZ] Fix trap issue and enable expensive checks.

The isBarrier/isTerminator flags have been removed from the SystemZ trap
instructions, so that tests do not fail with EXPENSIVE_CHECKS. This was just
an issue at -O0 and did not affect code output on benchmarks.

(Like Eli pointed out: "targets are split over whether they consider their
"trap" a terminator; x86, AArch64, and NVPTX don't, but ARM, MIPS, PPC, and
SystemZ do. We should probably try to be consistent here.". This is still the
case, although SystemZ has switched sides).

SystemZ now returns true in isMachineVerifierClean() :-)

These Generic tests have been modified so that they can be run with or without
EXPENSIVE_CHECKS: CodeGen/Generic/llc-start-stop.ll and
CodeGen/Generic/print-machineinstrs.ll

Review: Ulrich Weigand, Simon Pilgrim, Eli Friedman
https://bugs.llvm.org/show_bug.cgi?id=33047
https://reviews.llvm.org/D34143

llvm-svn: 306106