AMDGPU: Remove unnecessary string usage in AsmPrinter

Registers are printed a lot, so don't create temporary
std::strings. Using char instead of a string to an ostream
saves a function call.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@274581 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed