[CodeGenPrepare] Check that erased sunken address are not reused

CodeGenPrepare sinks address computations from one basic block to another
and attempts to reuse address computations that have already been sunk. If
the same address computation appears twice with the first instance as an
operand of a load whose result is an operand to a simplifable select,
CodeGenPrepare simplifies the select and recursively erases the now dead
instructions. CodeGenPrepare then attempts to use the erased address
computation for the second load.

Fix this by erasing the cached address value if it has zero uses before
looking for the address value in the sunken address map.

This partially resolves PR35209.

Thanks to Alexander Richardson for reporting the issue!

Reviewers: john.brawn

Differential Revision: https://reviews.llvm.org/D39841

llvm-svn: 318032