[analyzer] Don't use makeIntVal to create a floating-point value.

[analyzer] Don't use makeIntVal to create a floating-point value.

SimpleSValBuilder processes a couple trivial identities, including 'x - x'
and 'x ^ x' (both 0). However, the former could appear with arguments of
floating-point type, and we weren't checking for that. This started
triggering an assert with r163069, which checks that a constant value is
actually going to be used as an integer or pointer.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@163159 91177308-0d34-0410-b5e6-96231b3b80d8