[MCAsmStreamer] Print register names in --show-inst mode
Passing the context to `Inst.dump_pretty()` allows printing symbolic
register names instead of `<MCOperand Reg:1234>` in the output.
I plan to use this in a future RVY test cases where we have register
class with the same name in assembly syntax, but different underlying
register enum values. Printing the name of the enum value makes it
easier to test that we selected the correct register.
Reviewed By: lenary
Pull Request: https://github.com/llvm/llvm-project/pull/171252
