Fix assertion error
```
bool llvm::MachineOperand::isRenamable() const: Assertion `Register::isPhysicalRegister(getReg()) && "isRenamable should only be checked on physical registers"' failed.
```
by checking if the register is 0 before invoking `isRenamable`.
Summary:
This patch mainly do such transformation
```
$R0 = OP ...
... // No read/clobber of $R0 and $R1
$R1 = COPY $R0 // $R0 is killed
```
Replace $R0 with $R1 and remove the COPY, we have
```
$R1 = OP ...
```
This transformation can also expose more opportunities for existing
copy elimination in MCP.
Differential Revision: https://reviews.llvm.org/D67794
All tests with -run-pass !=none should not in MIR/, See MIR/README.
```
Tests for codegen passes should NOT be here but in
test/CodeGen/sometarget. As
a rule of thumb this directory should only contain tests using
'llc -run-pass none'.
```
llvm-svn: 371857
Kai Luo