[PowerPC] Remaining KnownBits should be constant when performing non-sign comparison

In `PPCTargetLowering::DAGCombineTruncBoolExt`, when checking if it's correct to perform the transformation for non-sign comparison, as the comment says
```
      // This is neither a signed nor an unsigned comparison, just make sure
      // that the high bits are equal.
```
Origin check
```
      if (Op1Known.Zero != Op2Known.Zero || Op1Known.One != Op2Known.One)
        return SDValue();
```
is not strong enough. For example,
```
Op1Known = 111x000x;
Op2Known = 111x000x;
```
Bit 4, besides bit 0, is still unknown and affects the final result.

This patch fixes https://bugs.llvm.org/show_bug.cgi?id=48388.

Reviewed By: nemanjai, #powerpc

Differential Revision: https://reviews.llvm.org/D93092