[llc] Use CPUStr instead of calling codegen::getMCPU(). NFC

`getCPUStr()` fallsback to `getMCPU()`.

The only difference between `getCPUStr()` and `getMCPU()` is that
`getCPUStr()` handles `-mcpu=native`. That doesn't matter for this case.

This is just a simplification of the original code and it does not
change the functionality. So no new tests added.

Differential Revision: https://reviews.llvm.org/D132849