[SimplifyCFG] Fix the compile crash for invalid upper bound value (#71351)

Fix the crash for the last land PR70542.

Note:
For '%add = add nuw i32 %x, 1', we can only infer the LowerBound is 1,
but the UpperBound is wrapped to 0 in computeConstantRange.
so we can't assume the UpperBound is valid bound when its value is 0.

Fix https://github.com/llvm/llvm-project/issues/71329.
Reviewed By: zmodem, nikic