[TIR] Support Return in TIR

[ZihengJiang]

This PR supports building TIR function directly and allows TIR returns value directly.
Current approach is by adding an intrinsic for return. Example:

def add():
    a = tir.Var("a", "float32") 
    b = tir.Var("b", "float32") 
    c = a + b
    c = tir.call_intrin("float32", "tir.myreturn", c) 
    c = tir.Evaluate(c)
    func = tir.PrimFunc([a, b], c)
    mod = tvm.IRModule({'add': func})
    func = tvm.build(mod['add'])
    out = func(1.0, 2.0)
    print(out)

In the future, with the tvm script, we should be able to write some like:

@tvm.script
def add(a, b):
    tir.myreturn(a + b)

There are two things need to be discussed:

• Because return is a keyword in Python, currently I use myreturn, we should have another name for it.
• Whether we should support multiple return in a single function.

@tqchen @junrushao1994 @areusch