[llvm] [InstCombine] Drop NSW when converting `shl X, BW - 1` back into mul (PR #121633)

[via llvm-commits]

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@@ -372,6 +372,32 @@ define <2 x i8> @srem_XY_XZ_with_CY_gt_CZ_no_nuw_out(<2 x i8> %X) {
   ret <2 x i8> %r
 }
 
+define i8 @srem_XY_XZ_with_CY_gt_CZ_drop_nsw(i8 noundef %X) {
+; CHECK-LABEL: @srem_XY_XZ_with_CY_gt_CZ_drop_nsw(
+; CHECK-NEXT:    [[BO0:%.*]] = mul nsw i8 [[X:%.*]], 127
+; CHECK-NEXT:    [[BO1:%.*]] = shl nsw i8 [[X]], 7
+; CHECK-NEXT:    [[R:%.*]] = srem i8 [[BO1]], [[BO0]]
+; CHECK-NEXT:    ret i8 [[R]]
+;
+  %BO0 = mul nsw i8 %X, 127
+  %BO1 = shl nsw i8 %X, 7
+  %r = srem i8 %BO1, %BO0
+  ret i8 %r
+}
+
+define i8 @srem_XY_XZ_with_CY_gt_CZ_drop_nsw_commuted(i8 noundef %X) {
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goldsteinn wrote:

Think its okay if the `shl` is the rhs of the `rem`:  https://alive2.llvm.org/ce/z/SoTDqo

https://github.com/llvm/llvm-project/pull/121633