[llvm] [InstCombine] Lower flag check pattern to use a bitmask-shift (PR #169557)

[Ryan Buchner via llvm-commits]

bababuck wrote:

Updated the PR description.
```
bool or_icmp(int type) {
  return (type == 0 || type == 6 || type == 15);
}
```
is converted to an IR switch statement during SimplifyCFG, which SimplifyCFG then transforms into a bitmask-shift, while
```
bool or_icmp(int type) {
  if (type == 0 || type == 6 || type == 15)
    return true;
  return false;
}
```
is not optimally lowered and is addressed by this MR (see https://compiler-explorer.com/z/9P7P7bj5r). From browsing, I don't think fixing this in `SimplifyCFG` is the best solution since this pattern can exist inside a single basic block (i.e. :
```
  %cmp = icmp eq i32 %type, 0
  %cmp1 = icmp eq i32 %type, 6
  %or.cond = or i1 %cmp, %cmp1
  %cmp3 = icmp eq i32 %type, 15
  %or.cond4 = or i1 %or.cond, %cmp3
  ret i1 %or.cond4
```

https://github.com/llvm/llvm-project/pull/169557