InstSimplify: Don't bother if getScalarSizeInBits returns zero

getScalarSizeInBits returns zero when the comparison operands are not
integral.  No functionality change intended.
InstCombine: Squash an icmp+select into bitwise arithmetic

(X & INT_MIN) == 0 ? X ^ INT_MIN : X  into  X | INT_MIN
(X & INT_MIN) != 0 ? X ^ INT_MIN : X  into  X & INT_MAX

This fixes PR21993.