[LV] Check if ops can safely be truncated in computeMinimumValueSizes.

Update computeMinimumValueSizes to check if an instruction's operands
can safely be truncated.

If more than MinBW bits are demanded by for the operand or if the
operand is a constant and cannot be safely truncated, it is not safe to
evaluate the instruction in the narrower MinBW. Skip those cases.

Fixes https://github.com/llvm/llvm-project/issues/47927

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D154717