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
When we find something like this:
```
%a:_(s32) = G_SOMETHING ...
...
%select:_(s32) = G_SELECT %cond(s1), %a, %a
```
We can remove the select and just replace it entirely with `%a` because it's
always going to result in `%a`.
Same if we have
```
%select:_(s32) = G_SELECT %cond(s1), %a, %b
```
where we can deduce that `%a == %b`.
This implements the following cases:
- `%select:_(s32) = G_SELECT %cond(s1), %a, %a` -> `%a`
- `%select:_(s32) = G_SELECT %cond(s1), %a, %some_copy_from_a` -> `%a`
- `%select:_(s32) = G_SELECT %cond(s1), %a, %b` -> `%a` when `%a` and `%b`
are defined by identical instructions
This gives a few minor code size improvements on CTMark at -O3 for AArch64.
Differential Revision: https://reviews.llvm.org/D76523
Jessica Paquette