[GlobalISel] Fix equality for copies from physregs in matchEqualDefs

When we see this:

```
%a = COPY $physreg
...
SOMETHING implicit-def $physreg
...
%b = COPY $physreg
```

The two copies are not equivalent, and so we shouldn't perform any folding
on them.

When we have two instructions which use a physical register check that they
define the same virtual register(s) as well.

e.g., if we run into this case

```
%a = COPY $physreg
...
%b = COPY %a
```

we can say that the two copies are the same, and can be folded.

Differential Revision: https://reviews.llvm.org/D76890