Remove verbose formatting of instructions on crash messages.

[jonmeow]

Undoes a chunk of #4125 because nobody's really in favor of keeping the formatting, and it's occasionally caused a crash in Formatter to dominate output (and even when working, it can be verbose; the source location in (5) is often more helpful).

Basically goes back to:

4.	Check::Context
          NodeStack:
            0. LetIntroducer: no value
            1. BindingPattern: inst+15
            2. LetInitializer: no value
            3. StructLiteralStart: no value
          inst_block_stack_:
            0.	block<invalid>	{inst+0, inst+1, inst+6, inst+7, inst+8, inst+9, inst+10, inst+11, inst+12}
            1.	global_init	{}
          pattern_block_stack_:
            0.	block<invalid>	{}
          param_and_arg_refs_stack:
            0.	block<invalid>	{}
          args_type_info_stack_:
            0.	block<invalid>	{}
5.	alias_of_alias.carbon:15:12: checking StructLiteral
          let d: c = {};
                     ^~

Fixes #4145