[ARM] don't transform an add(ext Cond), C to select unless there's a setcc of the condition

The transform in question claims to be doing:

// fold (add (select cc, 0, c), x) -> (select cc, x, (add, x, c))

...starting in PerformADDCombineWithOperands(), but it wasn't actually checking for a setcc node
for the sext/zext patterns.

This is exactly the opposite of a transform I'd like to add to DAGCombiner's foldSelectOfConstants(),
so I was seeing infinite loops with my draft of a patch applied.

The changes in select_const.ll look positive (less instructions). The change in arm-and-tst-peephole.ll
is unrelated. We're changing the input IR in that test to preserve the intent of the test, but that's 
not affected by this code change.

Differential Revision:
https://reviews.llvm.org/D30355

llvm-svn: 296389