[Preprocessor] Fix warning: left and right subexpressions are identical. NFCI.

This is reported by msvc as
warning C6287: redundant code: the left and right subexpressions are identical

EmittedDirectiveOnThisLine implies EmittedTokensOnThisLine
making this an NFC change. To be on the safe side and because both of
them are checked at other places as well, we continue to check both.

Compiler warning reported here:
https://reviews.llvm.org/D104601#2957333