[PowerPC] Remove implicit use register after transformToImmFormFedByLI()

When the instruction has imm form and fed by LI, we can remove the redundat LI instruction.
Below is an example:
```
    renamable $x5 = LI8 2
    renamable $x4 = exact SRD killed renamable $x4, killed renamable $r5, implicit $x5
```

will be converted to:
```
   renamable $x5 = LI8 2
   renamable $x4 = exact RLDICL killed renamable $x4, 62, 2,  implicit killed $x5
```

But when we do this optimization, we forget to remove implicit killed $x5
This bug has caused a lnt case error. This patch is to fix above bug.

Reviewed By: #powerpc, shchenz

Differential Revision: https://reviews.llvm.org/D85288