[InstCombine] Make `FoldOpIntoSelect` handle non-constants and use condition to deduce constants.

Make the fold use the information present in the condition for deducing constants i.e:
```
%c = icmp eq i8 %x, 10
%s = select i1 %c, i8 3, i8 2
%r = mul i8 %x, %s
```

If we fold the `mul` into the select, on the true side we insert `10` for `%x` in the `mul`.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D146349