[ValueTracking] Use select condition to help determine if select is non-zero
In `select c, x, y` the condition `c` dominates the resulting `x` or
`y` chosen by the `select`. This adds logic to `isKnownNonZero` to try
and use the `icmp` for the `c` condition to see if it implies the
select `x` or `y` are known non-zero.
For example in:
```
%c = icmp ugt i8 %x, %C
%r = select i1 %c, i8 %x, i8 %y
```
The true arm of select `%x` is non-zero (when "returned" by the
`select`) because `%c` being true implies `%x` is non-zero.
Alive2 Links (with `x {pred} C`):
- EQ iff `C != 0`:
- https://alive2.llvm.org/ce/z/umLabn
- NE iff `C == 0`:
- https://alive2.llvm.org/ce/z/DQvy8Y
- UGT [always]:
- https://alive2.llvm.org/ce/z/HBkjgQ
- UGE iff `C != 0`:
- https://alive2.llvm.org/ce/z/LDNifB
- SGT iff `C s>= 0`:
- https://alive2.llvm.org/ce/z/QzWDj3
- SGE iff `C s> 0`:
- https://alive2.llvm.org/ce/z/rR4g3D
- SLT iff `C s<= 0`:
- https://alive2.llvm.org/ce/z/uysayx
- SLE iff `C s< 0`:
- https://alive2.llvm.org/ce/z/2jYc7e
Reviewed By: nikic
Differential Revision: https://reviews.llvm.org/D147900
2622b2f409