[llvm] [MCP] Optimize copies when src is used during backward propagation (PR #111130)

[Quentin Colombet via llvm-commits]

qcolombet wrote:

> Is one of these test changes a dedicated MIR test for this situation?

Having a quick glance at the patch I don't think it is.

Could you add a few tests that checks specifically for the new pattern?
E.g.,
```
r0 = ..
// one of the use of r0 is not renameable
r1 = copy r0
```
and also
```
r0 = ..
// one of the use of r0 does not accept r1
r1 = copy r0
```
and so on.

Essentially add explicit test coverage with focused mir tests.

https://github.com/llvm/llvm-project/pull/111130