[LV] Check if compare is truncated directly in getInstructionCost.

The current check for truncated compares in getInstructionCost misses
cases where either the first or both operands are constants.
Check directly if the compare is marked for truncation. In that case,
the minimum bitwidth is that of the operands.

The patch also adds asserts to ensure that.

This fixes a divergence between legacy and VPlan-based cost model, where
the legacy cost model incorrectly estimated the cost of compares with
truncated operands.

Fixes https://github.com/llvm/llvm-project/issues/107171.

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