[VPlan] Handle truncating ICMPs in truncateToMinimalBWs.

Update truncateToMinimalBitwidths to handle truncating ICMPs. For ICMPs,
the new target type will be the same as the original type. In that case,
only truncate the operands, but skip the extend. This is in line with
what the original truncateToMinimalBitwidths did for compares.

Fixes https://github.com/llvm/llvm-project/issues/81415.