critical-anti-dependency breaker: don't use reg def info from kill insts (PR20308)

In PR20308 ( http://llvm.org/bugs/show_bug.cgi?id=20308 ), the critical-anti-dependency breaker
caused a miscompile because it broke a WAR hazard using a register that it thinks is available
based on info from a kill inst. Until PR18663 is solved, we shouldn't use any def/use info from
a kill because they are really just nops.

This patch adds guard checks for kills around calls to ScanInstruction() where the DefIndices
array is set. For good measure, add an assert in ScanInstruction() so we don't hit this bug again.

The test case is a reduced version of the code from the bug report.

Differential Revision: http://reviews.llvm.org/D4977