Rollup merge of #138280 - folkertdev:mir-dump-asm-const, r=compiler-errors

fix ICE in pretty-printing `global_asm!`

fixes rust-lang/rust#138260

since rust-lang/rust#137180, `global_asm!` gets a fake body, that the pretty printing logic did not know what to do with.

based on [#t-compiler/help > tests for MIR pretty printing](https://rust-lang.zulipchat.com/#narrow/channel/182449-t-compiler.2Fhelp/topic/tests.20for.20MIR.20pretty.20printing) I created `tests/ui/unpretty/mir` which seemed as good a place as any for a test. If there is a better place, let me know.

try-job: test-various
try-job: x86_64-apple-2

Author: Manishearth