[PatternMatch] Add support for matching ConstantExpr.

[huihuiz]

Summary

Motivated in D71389. For scalable vector type, constant folding
will be applied on ConstantExprs. It would be usefull to support
pattern matcher for ConstantExprs.

[efriedma]

It looks like we've always matched ConstantExprs for binary operators, but not for other operations. It probably makes sense to be more consistent.

If we're going to do this, probably makes sense to change OneOps_match/TwoOps_match at the same time.

Can you add testcases for a few of the existing transforms that are changing? There are too many transforms for it to make sense to cover everything, but it makes sense to illustrate what's actually changing.

[lebedev.ri]

In D71637#1788905, @efriedma wrote:

It looks like we've always matched ConstantExprs for binary operators, but not for other operations. It probably makes sense to be more consistent.

If we're going to do this, probably makes sense to change OneOps_match/TwoOps_match at the same time.

Can you add testcases for a few of the existing transforms that are changing? There are too many transforms for it to make sense to cover everything, but it makes sense to illustrate what's actually changing.

[lebedev.ri]

This review seems to be stuck/dead, consider abandoning if no longer relevant.

llvm/include/llvm/IR/PatternMatch.h

       : Op1(Op1), Op2(Op2), Op3(Op3) {}
 
   template <typename OpTy> bool match(OpTy *V) {
     if (V->getValueID() == Value::InstructionVal + Opcode) {
       auto *I = cast<Instruction>(V);
       return Op1.match(I->getOperand(0)) && Op2.match(I->getOperand(1)) &&
              Op3.match(I->getOperand(2));
     }
+    if (auto *CE = dyn_cast<ConstantExpr>(V))
+      return (CE->getOpcode() == Opcode) && Op1.match(CE->getOperand(0)) &&
+             Op2.match(CE->getOperand(1)) && Op3.match(CE->getOperand(2));
     return false;
   }
 };
 
 /// Matches SelectInst.
 template <typename Cond, typename LHS, typename RHS>
 inline ThreeOps_match<Cond, LHS, RHS, Instruction::Select>
 m_Select(const Cond &C, const LHS &L, const RHS &R) {