Make TableGen mangle operand method names before putting in enum.

The purpose of this patch is to allow PredicateMethods to be set to something
like "isUImm<8>", calling a C++ template method to reduce code duplication. For
this to work, the PredicateMethod must be mangled into a valid C++ identifier
for insertion into an enum.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@172073 91177308-0d34-0410-b5e6-96231b3b80d8