[llvm-mca] Refactor method RegisterFile::collectWrites(). NFCI

Rather than calling std::find in a loop, just sort the vector and remove
duplicate entries at the end of the function.

Also, move the debug print at the end of the function, and query the
MCRegisterInfo to print register names rather than physreg IDs.

No functional change intended.

llvm-svn: 335837