[InstCombine] ssubo X, C -> saddo X, -C

ssubo X, C is equivalent to saddo X, -C. Make the transformation in
InstCombine and allow the logic implemented for saddo to fold prior
usages of add nsw or sub nsw with constants.

Patch by Dan Robertson.

Differential Revision: https://reviews.llvm.org/D60061

llvm-svn: 358099