[InstCombine] reassociate fsub+fsub into fsub+fadd

As discussed in the motivating PR44509:
https://bugs.llvm.org/show_bug.cgi?id=44509

...we can end up with worse code using fast-math than without.
This is because the reassociate pass greedily transforms fsub
into fneg/fadd and apparently (based on the regression tests
seen here) expects instcombine to clean that up if it wasn't
profitable. But we were missing this fold:

(X - Y) - Z --> X - (Y + Z)

There's another, more specific case that I think we should
handle as shown in the "fake" fneg test (but missed with a real
fneg), but that's another patch. That may be tricky to get
right without conflicting with existing transforms for fneg.

Differential Revision: https://reviews.llvm.org/D72521