[MIPS][CallSiteInfo][NFC] Fill CallSiteInfo only when needed

[Prabhuk]

Summary

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.

[morehouse]

LGTM

[djtodoro]

llvm/lib/Target/Mips/MipsISelLowering.cpp

3352

I think we should fill the CallSiteInfo iff the Options.EmitCallSiteInfo is true only. The Options.SupportsDebugEntryValues is related to debug information that uses these call-site-info, but the CSInfo can be used for something else (even if the -g is not present; for example for the feature you are adding).
(Also, X86 and ARM fill that way).

[Prabhuk]

https://discourse.llvm.org/t/rfc-computing-storing-and-restoring-conservative-call-graphs-with-llvm/58446/3?u=prabhuk

[Prabhuk]

Rebased the patchset and addressed the compilation failures

llvm/lib/Target/Mips/MipsISelLowering.cpp

if (VA.isRegLoc()) {
 
       // If the parameter is passed through reg $D, which splits into
       // two physical registers, avoid creating call site info.
       if (Mips::AFGR64RegClass.contains(VA.getLocReg()))
         continue;
 
       // Collect CSInfo about which register passes which parameter.
       const TargetOptions &Options = DAG.getTarget().Options;
-      if (Options.SupportsDebugEntryValues)
+      if (Options.EmitCallSiteInfo && Options.SupportsDebugEntryValues)
         CSInfo.emplace_back(VA.getLocReg(), i);
 
       continue;
     }
 
     // Register can't get to this point...
     assert(VA.isMemLoc());