Remove SelfParam, add an AddrPattern instead. (#3506)

This is intended to make the representation of a `self` pattern be more
similar to other patterns.

toolchain/check/context.cpp

@@ -922,6 +922,7 @@ class TypeCompleter {
     // NOLINTNEXTLINE(bugprone-switch-missing-default-case)
     switch (inst.kind()) {
       case SemIR::AddressOf::Kind:
+      case SemIR::AddrPattern::Kind:
       case SemIR::ArrayIndex::Kind:
       case SemIR::ArrayInit::Kind:
       case SemIR::Assign::Kind:
@@ -954,7 +955,6 @@ class TypeCompleter {
       case SemIR::RealLiteral::Kind:
       case SemIR::Return::Kind:
       case SemIR::ReturnExpr::Kind:
-      case SemIR::SelfParam::Kind:
       case SemIR::SpliceBlock::Kind:
       case SemIR::StringLiteral::Kind:
       case SemIR::StructAccess::Kind: