[SystemZ] Call erase() on the right MBB in SystemZTargetLowering::emitSelect()

Since MBB was split *before* MI, the MI(s) will reside in JoinMBB (MBB) at
the point of erasing them, so calling StartMBB->erase() is actually wrong,
although it is "working" by all appearances.

Review: Ulrich Weigand
llvm-svn: 371995