[MIPS][CallSiteInfo][NFC] Fill CallSiteInfo only when needed (#86847)

Argument-register pairs in CallSiteInfo is only needed when
EmitCallSiteInfo
is on. Currently, the pairs are always pushed to the vector but only
used
when EmitCallSiteInfo is on.

Don't fill the CallSiteInfo vector unless used.

Differential Revision: https://reviews.llvm.org/D107108?id=362887



Co-authored-by: Necip Fazil Yildiran <necip@google.com>