Check usage for verbose formatting of instructions in crashes

[jonmeow]

#4125 added verbose formatting of instructions in crashes. If it's not used, or it gets in the way (a crash during a pretty printing) we might want to roll it back. This issue is to help track whether it's in use.

For example, this is the %<unexpected> formatted SemIR in the example crash output:

2.	Check::Context
          NodeStack:
(elided)
            2. IntLiteral:
              %<unexpected>.inst+26.loc12_10: i32 = int_literal 0 [template = constants.%.2]

Please comment here if you either:

1. Have a toolchain crash where you find the instruction formatting useful (it'd particularly help to explain why formatted was helpful)
   • Also if you have a crash where you expected it to be helpful, but it wasn't.
2. Have a toolchain crash which doesn't produce a stack trace, instead doing something like a SIGSEGV after argument printing and before Check::Context printing (which is symptomatic of a crash during the formatting, and could be worked around by just commenting it out temporarily).