Teach getZeroExtendExpr and getSignExtendExpr to use trip-count
information to simplify [sz]ext({a,+,b}) to {zext(a),+,[zs]ext(b)}, as appropriate. These functions and the trip count code each call into the other, so this requires careful handling to avoid infinite recursion. During the initial trip count computation, conservative SCEVs are used, which are subsequently discarded once the trip count is actually known. Among other benefits, this change lets LSR automatically eliminate some unnecessary zext-inreg and sext-inreg operation where the operand is an induction variable. llvm-svn: 70241
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