[PATCH] [mips] Fix forbidden slot hazard handling

MipsHazardSchedule has to determine what the next physical machine instruction
is to decide whether to insert a nop. In case where a branch with a forbidden
slot appears at the end of a basic block, first *real* instruction of the next
physical basic block was determined using getFirstNonDebugInstr().

Unfortunately this only considers DBG_VALUEs and not other transient opcodes
such as EHLABEL. As EHLABEL passes the SafeInForbiddenSlot predicate and the
instruction after the EHLABEL can be a CTI, we observed test failures in the
LNT testsuite.

Reviewers: dsanders

Differential Review: http://reviews.llvm.org/D19051

llvm-svn: 268052