[InstCombine] Refactor foldICmpMulConstant

This is a follow-up to 2ebfda2
(replace "if" with "else if" since the cases nuw/nsw
were meant to be handled separately).

Test plan:
1/ ninja check-llvm check-clang check-lld
2/ Bootstrapped LLVM/Clang pass tests

(cherry picked from commit d982f1e)

Author: alexander-shaposhnikov

llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp

@@ -2040,9 +2040,7 @@ Instruction *InstCombinerImpl::foldICmpMulConstant(ICmpInst &Cmp,
       NewC = ConstantInt::get(
           Mul->getType(),
           APIntOps::RoundingSDiv(C, *MulC, APInt::Rounding::DOWN));
-  }
-
-  if (Mul->hasNoUnsignedWrap()) {
+  } else if (Mul->hasNoUnsignedWrap()) {
     if (Pred == ICmpInst::ICMP_ULT || Pred == ICmpInst::ICMP_UGE)
       NewC = ConstantInt::get(
           Mul->getType(),

llvm/test/Transforms/InstCombine/icmp-mul.ll

@@ -860,7 +860,9 @@ define i1 @splat_mul_known_lz(i32 %x) {
 
 define i1 @splat_mul_unknown_lz(i32 %x) {
 ; CHECK-LABEL: @splat_mul_unknown_lz(
-; CHECK-NEXT:    [[R:%.*]] = icmp sgt i32 [[X:%.*]], -1
+; CHECK-NEXT:    [[Z:%.*]] = zext i32 [[X:%.*]] to i128
+; CHECK-NEXT:    [[M:%.*]] = mul nuw nsw i128 [[Z]], 18446744078004518913
+; CHECK-NEXT:    [[R:%.*]] = icmp ult i128 [[M]], 39614081257132168796771975168
 ; CHECK-NEXT:    ret i1 [[R]]
 ;
   %z = zext i32 %x to i128