[VectorCombine] Freeze index unless it is known to be non-poison.

If the index itself is already poison, the poison propagates through
instructions clamping the index to a valid range. This still causes
introducing a load of poison, as flagged by Alive2 and pointed out
at 575e2aff.

This patch updates the code to freeze the index, unless it is proven to
not be poison.

Reviewed By: nlopes

Differential Revision: https://reviews.llvm.org/D103378