[IR][InstCombine] Support scalable vector splats ConstantExprs in Constant::getUniqueInteger().

[craig.topper]

Summary

I've left the getAggregateElement as a fast path for non-ConstantExprs
to avoid a call to getSplatValue in release builds.

Fixes PR57989.

[spatel]

LGTM, but please add the fast path comment to the code too, so the explanation for this code structure is obvious.

llvm/lib/IR/Constants.cpp

 //===-- Constants.cpp - Implement Constant nodes --------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
if (OpC != Elt)
       return nullptr;
   }
   return Elt;
 }
 
 const APInt &Constant::getUniqueInteger() const {
   if (const ConstantInt *CI = dyn_cast<ConstantInt>(this))
     return CI->getValue();
+  // Scalable vectors can use a ConstantExpr to build a splat.
+  if (isa<ConstantExpr>(this))
+    return cast<ConstantInt>(this->getSplatValue())->getValue();
   assert(this->getSplatValue() && "Doesn't contain a unique integer!");
   const Constant *C = this->getAggregateElement(0U);
   assert(C && isa<ConstantInt>(C) && "Not a vector of numbers!");
   return cast<ConstantInt>(C)->getValue();
 }
 
 //---- ConstantPointerNull::get() implementation.
 //

llvm/test/Transforms/InstCombine/select.ll

 ; CHECK-NEXT:    ret i32 0
 ;
   %z = sext i1 %cond to i32
   %not_cond = xor i1 %cond, true
   %s = zext i1 %not_cond to i32
   %v = select i1 %cond, i32 %s, i32 %z
   ret i32 %v
 }
+
+; This previously crashed due to Constant::getUniqueInteger not handling
+; scalable vector splat ConstantExprs.
+define <vscale x 2 x i32> @and_constant_select_svec(<vscale x 2 x i32> %x, <vscale x 2 x i1> %cond) {
+; CHECK-LABEL: @and_constant_select_svec(
+; CHECK-NEXT:    [[A:%.*]] = and <vscale x 2 x i32> [[X:%.*]], shufflevector (<vscale x 2 x i32> insertelement (<vscale x 2 x i32> poison, i32 1, i32 0), <vscale x 2 x i32> poison, <vscale x 2 x i32> zeroinitializer)
+; CHECK-NEXT:    [[B:%.*]] = select <vscale x 2 x i1> [[COND:%.*]], <vscale x 2 x i32> [[A]], <vscale x 2 x i32> [[X]]
+; CHECK-NEXT:    ret <vscale x 2 x i32> [[B]]
+;
+  %a = and <vscale x 2 x i32> %x, shufflevector (<vscale x 2 x i32> insertelement (<vscale x 2 x i32> poison, i32 1, i32 0), <vscale x 2 x i32> poison, <vscale x 2 x i32> zeroinitializer)
+  %b = select <vscale x 2 x i1> %cond, <vscale x 2 x i32> %a, <vscale x 2 x i32> %x
+  ret <vscale x 2 x i32> %b
+}