[InstCombine] fix undef propagation bug with shuffle+binop

When we have a shuffle that extends a source vector with undefs
and then do some binop on that, we must make sure that the extra
elements remain undef with that binop if we reverse the order of
the binop and shuffle.

'or' is probably the easiest example to show the bug because
'or C, undef --> -1' (not undef). But there are other 
opcode/constant combinations where this is true as shown by 
the 'shl' test.

llvm-svn: 348191