add a new OPC_SwitchOpcode which is semantically equivalent

add a new OPC_SwitchOpcode which is semantically equivalent
to a scope where every child starts with a CheckOpcode, but
executes more efficiently.  Enhance DAGISelMatcherOpt to
form it.

This also fixes a bug in CheckOpcode: apparently the SDNodeInfo
objects are not pointer comparable, we have to compare the
enum name.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@97438 91177308-0d34-0410-b5e6-96231b3b80d8