[PATCH] Distribute binary operators through SELECT_CC

[Richard Sandiford]

This patch transforms something like:

  (add (select_cc X, Y, 10, 20, CC), 10)

into:

  (select_cc X, Y, 20, 30, CC)

but generalised for a tree of select_ccs.  It allows at most one of
the selected values to be non-constant.

If this goes in, I'd like to follow it with patches to handle unary
operations and floating-point binary operations in the same way.

The main motivation was to optimise operations on scalarised vector
selects.  It also changed the output of the R600/alu-shift.ll test that
Tom removed earlier.  Tom reckoned the new code was an improvement
(but defeated the original purpose of that test).

Thanks,
Richard

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