[clang] [CIR] Implement switch case simplify (PR #140649)

[via cfe-commits]

Andres-Salamanca wrote:

@andykaylor @erichkeane @bcardosolopes 
While implementing this simplification, I noticed a difference compared to the previous behavior where the folding happened during codegen.

Consider the following test case:

```cpp
void sw7(int a) {
  switch (a) {
  case 0:
  case 1:
  case 2:
    int x;
  case 3:
  case 4:
  case 5:
    break;
  }
}
```
In the [previous implementation](https://github.com/llvm/clangir/blob/1fb346d9c05fa676b76f512eded5bba4010b4a74/clang/test/CIR/CodeGen/switch.cpp#L145), the codegen phase performed the folding. Variable `x` was **hoisted outside** the switch and the generated CIR looked like this:
```
 cir.case(anyof, [#cir.int<0> : !s32i, #cir.int<1> : !s32i, #cir.int<2> : !s32i]) {
   cir.yield
 }
 cir.case(anyof, [#cir.int<3> : !s32i, #cir.int<4> : !s32i, #cir.int<5> : !s32i]) {
   cir.break
 }
```
Now, without codegen folding and before applying the new pass, the output is:

```mlir
%1 = cir.alloca !s32i, !cir.ptr<!s32i>, ["x"] {alignment = 4 : i64} // x hoisted out as before
%2 = cir.load %0 : !cir.ptr<!s32i>, !s32i
cir.switch (%2 : !s32i) {
  cir.case(equal, [#cir.int<0> : !s32i]) {
    cir.yield
  }
  cir.case(equal, [#cir.int<1> : !s32i]) {
    cir.yield
  }
  cir.case(equal, [#cir.int<2> : !s32i]) {
    cir.yield
  }
  cir.case(equal, [#cir.int<3> : !s32i]) {
    cir.yield
  }
  cir.case(equal, [#cir.int<4> : !s32i]) {
    cir.yield
  }
  cir.case(equal, [#cir.int<5> : !s32i]) {
    cir.break
  }
  cir.yield
}
```
After applying the new CIR simplify pass, the output is:

```mlir
cir.case(anyof, [#cir.int<0> : !s32i, #cir.int<1> : !s32i, #cir.int<2> : !s32i, #cir.int<3> : !s32i, #cir.int<4> : !s32i, #cir.int<5> : !s32i]) {
   cir.break
}
```

https://github.com/llvm/llvm-project/pull/140649