[LoopReroll] Allow for multiple loop control only induction vars

Before this, LoopReroll would fail an assertion, falsely assuming that
there can only possibly a single loop control only induction variable.

For example:

```
%a = phi i16 [ %dec2, %for.body ], [ 0, %entry ]
%b = phi i16 [ %dec1, %for.body ], [ 0, %entry ]
%a.next = add nsw i16 %1, -1
%b.next = add nsw i16 %0, -1
%add = add nsw i16 %a, %b
; ... rerollable code
%cmp.not = icmp eq i16 -10, %add
br i1 %cmp.not, label %exit, label %loop
```

Both %a and %b are valid loop control only induction vars

Additionally, some NFC changes to remove unnecessary isa<PHINode> check

Updated complex_reroll checks

Differential Revision: https://reviews.llvm.org/D141109