MIR Parser: Extract the code that parses the alignment into a new method. NFC.

MIR Parser: Extract the code that parses the alignment into a new method. NFC.

This commit extracts the code that parses the memory operand's alignment into
a new method named 'parseAlignment' so that it can be reused when parsing the
basic block's alignment attribute.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@244945 91177308-0d34-0410-b5e6-96231b3b80d8