[Transforms] Enhance CorrelatedValuePropagation to handle both values of select

The "Correlated Value Propagation" pass was missing a case when handling select instructions. It was only handling the "false" constant value, while in NVPTX the select may have the condition (and thus the branches) inverted, for example:
```
loop:
	%phi = phi i32* [ %sel, %loop ], [ %x, %entry ]
	%f = tail call i32* @f(i32* %phi)
	%cmp1 = icmp ne i32* %f, %y
	%sel = select i1 %cmp1, i32* %f, i32* null
	%cmp2 = icmp eq i32* %sel, null
	br i1 %cmp2, label %return, label %loop
```
But the select condition can be inverted:
```
	%cmp1 = icmp eq i32* %f, %y
	%sel = select i1 %cmp1, i32* null, i32* %f
```
The fix is to enhance "Correlated Value Propagation" to handle both branches of the select instruction.

Reviewed By: nikic, lebedev.ri

Differential Revision: https://reviews.llvm.org/D119643