MIR Parser: Extract the code that parses stack object references into a new

MIR Parser: Extract the code that parses stack object references into a new
method.

This commit extracts the code that parses the stack object references into a
new method named 'parseStackFrameIndex', so that it can be reused when
parsing standalone stack object references.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@245370 91177308-0d34-0410-b5e6-96231b3b80d8