[InstCombine] FP: Z - (X - Y) --> Z + (Y - X)

This restores what was lost with rL73243 but without
re-introducing the bug that was present in the old code.

Note that we already have these transforms if the ops are 
marked 'fast' (and I assume that's happening somewhere in
the code added with rL170471), but we clearly don't need 
all of 'fast' for these transforms.

llvm-svn: 329362