[TableGen][ISel] Do a better job of factoring ScopeMatchers created during creation...

[TableGen][ISel] Do a better job of factoring ScopeMatchers created during creation of SwitchTypeMatcher.

Previously we were factoring when the ScopeMatcher was initially created, but it might get more Matchers added to it later. Delay factoring until we have fully created/populated the ScopeMatchers.

This reduces X86 isel tables by 154 bytes.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@287520 91177308-0d34-0410-b5e6-96231b3b80d8