[SCEV][NFC] Remove check for rewriteable types

[mkazantsev]

Summary

I guess its only reason to exist is potential CT optimization, otherwise it is
just creating cohesion between this code and rewriter internals. We plan to
extend the rewriter. I'd rather not have this cohesion, unless there is a serious
reason to have it.

[mkazantsev]

@nikic is it a big deal if we remove it? I mean, CT.

[mkazantsev]

I guess I can use https://llvm-compile-time-tracker.com/ to know CT impact and revert it if it's visible.

[fhahn]

I put the whole stack on the compile-time tracker. This is for the patch: https://llvm-compile-time-tracker.com/compare.php?from=4f01266954b12395add2a6a87ef589126cb147fa&to=a86ad269a56d52099c9305738f5b20809b8dc7a5&stat=instructions:u

One notable change is +0.11% for mafft, but it is still relatively small. Numbers for all changes should be available under fhahn/perf/scev-expr-rewriter

llvm/lib/Analysis/ScalarEvolution.cpp

if (isa<SCEVConstant>(LHS) || containsAddRecurrence(RHS))
       return;
 
     // If RHS is SCEVUnknown, make sure the information is applied to it.
     if (!isa<SCEVUnknown>(LHS) && isa<SCEVUnknown>(RHS)) {
       std::swap(LHS, RHS);
       Predicate = CmpInst::getSwappedPredicate(Predicate);
     }
 
-    // Limit to expressions that can be rewritten.
-    if (!isa<SCEVUnknown>(LHS) && !isa<SCEVZeroExtendExpr>(LHS))
-      return;
-
     // Check whether LHS has already been rewritten. In that case we want to
     // chain further rewrites onto the already rewritten value.
     auto I = RewriteMap.find(LHS);
     const SCEV *RewrittenLHS = I != RewriteMap.end() ? I->second : LHS;
 
     const SCEV *RewrittenRHS = nullptr;
     switch (Predicate) {
     case CmpInst::ICMP_ULT: {