[ConstantRange] Add isAllNegative() and isAllNonNegative() methods

Add isAllNegative() and isAllNonNegative() methods to ConstantRange,
which determine whether all values in the constant range are
negative/non-negative.

This is useful for replacing KnownBits isNegative() and isNonNegative()
calls when changing code to use constant ranges.

Differential Revision: https://reviews.llvm.org/D60264

llvm-svn: 357871