Teach the DAGCombine to fold this pattern(c1 and c2 is constant).

// fold (sext (select cond, c1, c2)) -> (select cond, sext c1, sext c2)
// fold (zext (select cond, c1, c2)) -> (select cond, zext c1, zext c2)
// fold (aext (select cond, c1, c2)) -> (select cond, sext c1, sext c2)
Sign extend the operands if it is any_extend, to keep the signess of the operands that, the other combine rule would apply. The any_extend is handled as zero extend for constants. i.e.

t1: i8 = select t0, Constant:i8<-1>, Constant:i8<0>
t2: i64 = any_extend t1
 -->
t3: i64 = select t0, Constant:i64<-1>, Constant:i64<0>
 -->
t4: i64 = sign_extend_inreg t3

Differential Revision: https://reviews.llvm.org/D63318

llvm-svn: 364382