MIR Parser: Extract the IR constant parsing code into a new method. NFC.

MIR Parser: Extract the IR constant parsing code into a new method. NFC.

This commit extracts the code that parses the IR constant values into a new
method named 'parseIRConstant' in the 'MIParser' class. The new method will
be reused by the code that parses the typed integer immediate machine operands.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@244093 91177308-0d34-0410-b5e6-96231b3b80d8