MC: Cleanup parseMSInlineAsm

Utilize range based for-loops to simplify some code.
Use insert() instead of a loop for simplicity/efficiency.

No functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211486 91177308-0d34-0410-b5e6-96231b3b80d8