[SCEV] Fix BinomialCoefficient Iteration to fit in W bits

BinomialCoefficient computes the value of W-bit IV at iteration It of a
loop. When W is 1, we can call multiplicative inverse on 0 which
triggers an assert since 1b76120.

Since the arithmetic is supposed to wrap if It or K does not fit in W
bits, do the truncation into W bits after we do the shift.

Fixes #87798

Author: annamthomas

llvm/lib/Analysis/ScalarEvolution.cpp

@@ -928,10 +928,9 @@ static const SCEV *BinomialCoefficient(const SCEV *It, unsigned K,
   APInt OddFactorial(W, 1);
   unsigned T = 1;
   for (unsigned i = 3; i <= K; ++i) {
-    APInt Mult(W, i);
-    unsigned TwoFactors = Mult.countr_zero();
+    unsigned TwoFactors = countr_zero(i);
     T += TwoFactors;
-    Mult.lshrInPlace(TwoFactors);
+    APInt Mult(W, i >> TwoFactors);
     OddFactorial *= Mult;
   }
 

llvm/test/Transforms/IndVarSimplify/pr87798.ll

@@ -1,7 +1,6 @@
 ; RUN: opt -S -passes='indvars' -verify-scev < %s | FileCheck %s
 
 ; REQUIRES: asserts
-; XFAIL: *
 
 target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128-ni:1-p2:32:8:8:32-ni:2"
 target triple = "x86_64-unknown-linux-gnu"