[ConstantRange] Disallow NUW | NSW in makeGuaranteedNoWrapRegion()

As motivated in D60598, this drops support for specifying both NUW and
NSW in makeGuaranteedNoWrapRegion(). None of the users of this function
currently make use of this.

When both NUW and NSW are specified, the exact nowrap region has two
disjoint parts and makeGNWR() returns one of them. This result doesn't
seem to be useful for anything, but makes the semantics of the function
fuzzier.

Differential Revision: https://reviews.llvm.org/D60632

llvm-svn: 358340