[RDA] Fix DBG_VALUE issues

We skip debug instructions in RDA so we cannot attempt to look them
up in our instruction map without causing a crash. But some of the
methods select the last instruction in the block and this
instruction may be a debug instruction... So, use getLastNonDebugInstr
instead of calling back on a MachineBasicBlock.

MachineBasicBlock iterators have also been updated to use
instructionsWithoutDebug so we can avoid the manual checks for debug
instructions.

Differential Revision: https://reviews.llvm.org/D85658