[ConstantRange] Fix single bit abs range (PR59887)

For a full range input, we would produce an empty range instead
of a full range. The change to the SMin.isNonNegative() branch is
an optimality fix, because we should account for the potentially
discarded SMin value in the IntMinIsPoison case.

Change TestUnaryOpExhaustive to test both 4 and 1 bits, to both
cover this specific case in unit tests, and make sure all other
unary operations deal with 1-bit inputs correctly.

Fixes https://github.com/llvm/llvm-project/issues/59887.