[CIR] Upstream support for loops

[andykaylor]

Overview

Upstream support for various simple loops (for, while, do) the cir.for, cir.do, cir.while, cir.break, and cir.continue operations.

Suggested minimal test case

void f() {
  for (;;)
    ;
}

Existing incubator tests

clang/test/CIR/CodeGen/loop.cpp

[andykaylor]

A couple of pieces of this have landed

For loops
#132266

Do and While loops
#133157

I'll be posting a PR soon for break and continue statements