[NFC][MIR] Fix extra whitespace in MIR printing (#162928)
Fix a whitespace regression in MIR printing that was introduced in
https://github.com/llvm/llvm-project/pull/137361.
The default value for `ListSeparator` is `", "`, so we don't need to
print an additional space in front of tokens for optional symbols and
other things printed after operands.
Note, the modified LIT test will fail at trunk without the fix,
demonstrating that the extra space before `, pre-instr-symbol <mcsymbol
>` on Line 63 exists currently and is fixed with this change.
