fix CastInst::castIsValid to reject aggregate types, fixing PR6153:

llvm-as: t.ll:1:25: error: invalid cast opcode for cast from '[4 x i8]' to '[1 x i32]'
@x = constant [1 x i32] bitcast ([4 x i8] c"abcd" to [1 x i32])
                        ^

llvm-svn: 94595