[PATCH] Fix write-back value propagation for pre-indexed addressing modes

[Silviu Baranga]

Hi,

The attached patch fixes a bug in DAG combiner that causes a wrong usage
of the value write-back value obtained from pre-indexed addressing modes.

The bug manifests itself when the base pointer and the offset are swapped
in a load/store store instruction with a pre decrement addressing mode.

If a user needs to subtract the base from a constant value, the current
code re-writes the expression as a SUB instruction, subtracting the write-back
value from an adjusted constant. This is wrong, since the correct output
is to add an adjusted constant to the write-back value.

Since the code is apparently impossible to test, the patch does not contain a
regression test. Because of this I've re-written the code that handles the
propagation of the write-back value so that it is easy to follow and unlikely
to have fundamental bugs in it.

Please review!

Thanks,
Silviu

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