[analyzer] Don't attempt to create a floating-point value of "1" for ++/--.

The current logic would actually create a float- or double-sized signed
integer value of 1, which is not at all the same.

No test because the value would be swallowed by an Unknown as soon as it
gets added or subtracted to the original value, but it enables the cleanup
in the next patch.

llvm-svn: 163068