InstCombine: Fix a potential bug in 0 - (X sdiv C) -> (X sdiv -C)

While *most* (X sdiv 1) operations will get caught by InstSimplify, it
is still possible for a sdiv to appear in the worklist which hasn't been
simplified yet.

This means that it is possible for 0 - (X sdiv 1) to get transformed
into (X sdiv -1); dividing by -1 can make the transform produce undef
values instead of the proper result.

Sorry for the lack of testcase, it's a bit problematic because it relies
on the exact order of operations in the worklist.

llvm-svn: 215818