[MIRParser] Update a diagnostic message to use the correct register sigil. NFC

Summary:
Patch r323922 changed the sigil for physical registers to '$',  instead of '%'.
An error message was missed during this change, and reports the wrong sigil.
This patch corrects that diagnostic and the tests that check that error string.


Reviewers: zer0, bjope

Reviewed By: bjope

Subscribers: bjope, thegameg, plotfi, llvm-commits

Differential Revision: https://reviews.llvm.org/D48086

llvm-svn: 335066