[3/6] ARM Neon Intrinsic Tablegen Test Generator.

Refactored out the method InstructionTypeCode from MangleName for use in
further patches which perform neon tablegen test generation.

Reviewed by Bob Wilson.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@179636 91177308-0d34-0410-b5e6-96231b3b80d8