[NFC][InstSimplify] Add tests for add nuw %x, -1 -> -1 fold.

%ret = add nuw i8 %x, C
From langref:
	nuw and nsw stand for “No Unsigned Wrap” and “No Signed Wrap”,
	respectively. If the nuw and/or nsw keywords are present,
	the result value of the add is a poison value if unsigned
	and/or signed overflow, respectively, occurs.

So if C is -1, %x can only be 0, and the result is always -1.

https://rise4fun.com/Alive/sldC
Was mentioned in D47428 review.

llvm-svn: 334236