[Opt] Simplify multiplying/dividing POT

[xumingkuan]

Related issue = #944 #656 #2177

This PR may help with some optimizations after #2327 is merged. When we want to access a leaf (place) SNode and (the same address of) its parent or some ancestor, simple optimizations such as CSE can remove the duplicated part of the access paths after lower_access when nonconsecutive indices are used (now). However, when consecutive indices are used, we may have some code like this:

a = ti.field(ti.i32)
block = root.dense(ti.i, 64)
block.dense(ti.i, 3).place(a)

@ti.kernel
def foo():
  for i in ...:
    ti.activate(block, i / 8)
    a[i]...

And we want the common part of block[i / 8] and a[i] is reused in the two accesses. After expanding the BitExtractStmt, the former one will be (i / 8) & 63 and the latter one will be (i >> 3) & 63. This PR replaces i / 8 with i >> 3, and further replaces (a >> b) >> c with a >> (b + c) when both b and c are constants, so that CSE will be able to remove the duplicated access paths after lower_access.

This PR also enables constant folding for unsigned data types.

[Click here for the format server]

---

[k-ye]

LGTM!