[IR] Creation-time constant fold for constant expressions

[yaoyaoding]

Previously, when we do some arhtimatic operations for constant, we will keep them in our IR:

from hidet.ir.expr import convert
a = convert(1)
b = convert(2)
c = a + b
print(c)  # would be Add(Constant(1), Constant(2))

With this PR, we will do the computation when we create the Add expression, and replace it with hidet.ir.Constant:

from hidet.ir.expr import convert
a = convert(1)
b = convert(2)
c = a + b
print(c)  # would be Constant(3)

This PR also clean the hidet.ir.dialects.pattern.py. We do not many fancy pattern matching any more.