[Reassociate] Don't canonicalize x + (-Constant * y) -> x - (Constant * y)..

..if the resulting subtract will be broken up later.  This can cause us to get
into an infinite loop.

x + (-5.0 * y)      -> x - (5.0 * y)       ; Canonicalize neg const
x - (5.0 * y)       -> x + (0 - (5.0 * y)) ; Break up subtract
x + (0 - (5.0 * y)) -> x + (-5.0 * y)      ; Replace 0-X with X*-1.

PR34078

llvm-svn: 311554