[IndVarSimplify] Fix poison-safety when reusing instructions (#80458)

IndVars may replace an instruction with one of its operands, if they
have the same SCEV expression. However, such a replacement may be more
poisonous.

First, check whether the operand being poison implies that the
instruction is also poison, in which case the replacement is always
safe. If this fails, check whether SCEV can determine that reusing the
instruction is safe, using the same check as SCEVExpander.

Fixes https://github.com/llvm/llvm-project/issues/79861.

(cherry picked from commit 7d2b6f0b)