To find the exiting VN of a LiveInterval from a block, use the previous slot

rather than the previous index. If a block has a single instruction, the
previous index may be in a different basic block.

I have no clue how this used to work on all of test-suite, because now this
failure is seen quite often when trying to compile code with -strong-phi-elim.
This fixes PR10252.

llvm-svn: 141812