[SCEV] Use ashr to adjust constant multipliers (#135534)

SCEV converts "-2 *nsw (i32 V)" into "2148473647 *nsw (i32 V)". But we
cannot preserve the nsw flag when the constant multiplier is negative.
This patch changes lshr to ashr so that we can preserve both nsw and nuw
flags.

Alive2 proof: https://alive2.llvm.org/ce/z/LZVSEa
Closes #135531.

Author: dtcxzyw

llvm/lib/Analysis/ScalarEvolution.cpp

@@ -7841,7 +7841,7 @@ const SCEV *ScalarEvolution::createSCEV(Value *V) {
               unsigned GCD = std::min(MulZeros, TZ);
               APInt DivAmt = APInt::getOneBitSet(BitWidth, TZ - GCD);
               SmallVector<const SCEV*, 4> MulOps;
-              MulOps.push_back(getConstant(OpC->getAPInt().lshr(GCD)));
+              MulOps.push_back(getConstant(OpC->getAPInt().ashr(GCD)));
               append_range(MulOps, LHSMul->operands().drop_front());
               auto *NewMul = getMulExpr(MulOps, LHSMul->getNoWrapFlags());
               ShiftedLHS = getUDivExpr(NewMul, getConstant(DivAmt));

llvm/test/Analysis/ScalarEvolution/pr135531.ll

@@ -0,0 +1,19 @@
+; NOTE: Assertions have been autogenerated by utils/update_analyze_test_checks.py UTC_ARGS: --version 5
+; RUN: opt -disable-output -passes='print<scalar-evolution>' < %s 2>&1 | FileCheck %s
+
+define i32 @pr135511(i32 %x) {
+; CHECK-LABEL: 'pr135511'
+; CHECK-NEXT:  Classifying expressions for: @pr135511
+; CHECK-NEXT:    %and = and i32 %x, 16382
+; CHECK-NEXT:    --> (2 * (zext i13 (trunc i32 (%x /u 2) to i13) to i32))<nuw><nsw> U: [0,16383) S: [0,16383)
+; CHECK-NEXT:    %neg = sub nsw i32 0, %and
+; CHECK-NEXT:    --> (-2 * (zext i13 (trunc i32 (%x /u 2) to i13) to i32))<nsw> U: [0,-1) S: [-16382,1)
+; CHECK-NEXT:    %res = and i32 %neg, 268431360
+; CHECK-NEXT:    --> (4096 * (zext i16 (trunc i32 ((-1 * (zext i13 (trunc i32 (%x /u 2) to i13) to i32))<nsw> /u 2048) to i16) to i32))<nuw><nsw> U: [0,268431361) S: [0,268431361)
+; CHECK-NEXT:  Determining loop execution counts for: @pr135511
+;
+  %and = and i32 %x, 16382
+  %neg = sub nsw i32 0, %and
+  %res = and i32 %neg, 268431360
+  ret i32 %res
+}