[InstCombine] Don't limit uses in eraseInstFromFunction()

eraseInstFromFunction() adds the operands of the erased instructions,
as those might now be dead as well. However, this is limited to
instructions with less than 8 operands.

This check doesn't make a lot of sense to me. As the instruction
gets removed afterwards, I don't see a potential for anything
overly pathological happening here (as we can only add those
operands to the worklist once). The impact on CTMark is in
the noise. We also have the same code in instruction sinking
and don't limit the operand count there.

Differential Revision: https://reviews.llvm.org/D77325