[MIR] Fix CFI_INSTRUCTION escape printing

Summary:
The printer seems to intend to not print the trailing comma but has a
copy-paste error for the last value in the escape, and the parser
enforces having no trailing comma, but somehow a test was never included
to actually confirm it.

Reviewers: thegameg, arsenm

Reviewed By: thegameg, arsenm

Subscribers: wdng, arsenm, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D82478