[MCP] Do not try forward non-existent sub-register of a copy

In this example:
```
$d14 = COPY killed $d18
$s0 = MI $s28
```

$s28 is a sub-register of $d14. However, $d18 does not have
sub-registers and thus cannot be forwarded. Previously, this resulted
in $noreg being substituted in place of the use of $s28, which later
led to an assertion failure.

Fixes https://github.com/llvm/llvm-project/issues/60908, a regression
that was introduced in D141747.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D146930