[SimplifyCFG] Use range check in simplifyBranchOnICmpChain if possible (#165105)
In `simplifyBranchOnICmpChain`, if we can merge the comparisons into a
range check, use a conditional branch instead. This change also breaks
the cycle found in https://github.com/llvm/llvm-project/issues/165088.
Closes https://github.com/llvm/llvm-project/issues/165088.
Detailed description of the cycle:
```
define void @pr165088_cycle_1(i8 %x) {
entry:
%switch = icmp uge i8 %x, 2
%cond1 = icmp ugt i8 %x, 1
%or.cond = and i1 %switch, %cond1
br i1 %or.cond, label %block3, label %block2
block1:
%cond2 = icmp ugt i8 %x, 1
br i1 %cond2, label %block3, label %block2
block2:
br label %block3
block3:
%cond3 = icmp eq i8 %x, 0
br i1 %cond3, label %exit, label %block1
exit:
ret void
}
```
`simplifyBranchOnICmpChain` folds the branch in `entry` to a switch.
Then we get:
```
entry:
switch i8 %x, label %block3 [
i8 1, label %block2
i8 0, label %block2
]
...
```
`performValueComparisonIntoPredecessorFolding` redirects the default
target `block3` into `block1` because `%x` is never zero.
```
entry:
switch i8 %x, label %block1 [
i8 1, label %block2
i8 0, label %block2
]
...
```
Then `turnSwitchRangeIntoICmp` will convert the switch back into a
branch on icmp.
```
entry:
%switch = icmp ult i8 %x, 2
br i1 %switch, label %block2, label %block1
...
```
Since `block1` and `block2` share the same successor `block3`,
`performBranchToCommonDestFolding` merges the conditions of `entry` and
`block1`, resulting in the original pattern again.
llvm-project
e8a1162b - [SimplifyCFG] Use range check in simplifyBranchOnICmpChain if possible (#165105)
