[Stackmap] Refactor operand parsing.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197329 91177308-0d34-0410-b5e6-96231b3b80d8
Replace string matching with a switch on Triple::getEnvironment.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197332 91177308-0d34-0410-b5e6-96231b3b80d8