Directly convert from an initializer to a value where possible. (#3306)

If the initializing representation is the same as the value
representation, don't materialize a temporary and perform a value
binding. Instead, directly extract the value, using a new
`value_of_initializer` node.

This removes a lot of redundant `alloca`s from our generated LLVM IR.

toolchain/check/context.cpp

@@ -695,6 +695,7 @@ class TypeCompleter {
       case SemIR::TupleValue::Kind:
       case SemIR::UnaryOperatorNot::Kind:
       case SemIR::ValueAsReference::Kind:
+      case SemIR::ValueOfInitializer::Kind:
       case SemIR::VarStorage::Kind:
         CARBON_FATAL() << "Type refers to non-type node " << node;