[InstSimplify] (-1 << x) s>> x --> -1

This was noticed in:
https://llvm.org/PR51351

https://alive2.llvm.org/ce/z/aLxunD

llvm/lib/Analysis/InstructionSimplify.cpp

@@ -1429,9 +1429,11 @@ static Value *SimplifyAShrInst(Value *Op0, Value *Op1, bool isExact,
                                     MaxRecurse))
     return V;
 
-  // all ones >>a X -> -1
+  // -1 >>a X --> -1
+  // (-1 << X) a>> X --> -1
   // Do not return Op0 because it may contain undef elements if it's a vector.
-  if (match(Op0, m_AllOnes()))
+  if (match(Op0, m_AllOnes()) ||
+      match(Op0, m_Shl(m_AllOnes(), m_Specific(Op1))))
     return Constant::getAllOnesValue(Op0->getType());
 
   // (X << A) >> A -> X

llvm/test/Transforms/InstCombine/partally-redundant-left-shift-input-masking-after-truncation-variant-d.ll

@@ -246,17 +246,12 @@ define i32 @n6_extrause2(i64 %x, i32 %nbits) {
   ret i32 %t6
 }
 
-; TODO: shl+ashr of -1 should be reducecd.
 ; This is a miscompile if it ends by masking off the high bit of the result.
 
 define i32 @PR51351(i64 %x, i32 %nbits) {
 ; CHECK-LABEL: @PR51351(
-; CHECK-NEXT:    [[T0:%.*]] = zext i32 [[NBITS:%.*]] to i64
-; CHECK-NEXT:    [[T1:%.*]] = shl i64 -1, [[T0]]
-; CHECK-NEXT:    [[T2:%.*]] = ashr i64 [[T1]], [[T0]]
-; CHECK-NEXT:    [[T3:%.*]] = add i32 [[NBITS]], -33
-; CHECK-NEXT:    [[T4:%.*]] = and i64 [[T2]], [[X:%.*]]
-; CHECK-NEXT:    [[T5:%.*]] = trunc i64 [[T4]] to i32
+; CHECK-NEXT:    [[T3:%.*]] = add i32 [[NBITS:%.*]], -33
+; CHECK-NEXT:    [[T5:%.*]] = trunc i64 [[X:%.*]] to i32
 ; CHECK-NEXT:    [[T6:%.*]] = shl i32 [[T5]], [[T3]]
 ; CHECK-NEXT:    ret i32 [[T6]]
 ;

llvm/test/Transforms/InstSimplify/shift.ll

@@ -288,9 +288,7 @@ define i32 @poison6(i32 %x) {
 
 define i32 @all_ones_left_right(i32 %x) {
 ; CHECK-LABEL: @all_ones_left_right(
-; CHECK-NEXT:    [[LEFT:%.*]] = shl i32 -1, [[X:%.*]]
-; CHECK-NEXT:    [[RIGHT:%.*]] = ashr i32 [[LEFT]], [[X]]
-; CHECK-NEXT:    ret i32 [[RIGHT]]
+; CHECK-NEXT:    ret i32 -1
 ;
   %left = shl i32 -1, %x
   %right = ashr i32 %left, %x
@@ -299,26 +297,26 @@ define i32 @all_ones_left_right(i32 %x) {
 
 define <2 x i7> @all_ones_left_right_splat(<2 x i7> %x) {
 ; CHECK-LABEL: @all_ones_left_right_splat(
-; CHECK-NEXT:    [[LEFT:%.*]] = shl <2 x i7> <i7 -1, i7 -1>, [[X:%.*]]
-; CHECK-NEXT:    [[RIGHT:%.*]] = ashr <2 x i7> [[LEFT]], [[X]]
-; CHECK-NEXT:    ret <2 x i7> [[RIGHT]]
+; CHECK-NEXT:    ret <2 x i7> <i7 -1, i7 -1>
 ;
   %left = shl <2 x i7> <i7 -1, i7 -1>, %x
   %right = ashr <2 x i7> %left, %x
   ret <2 x i7> %right
 }
 
+; Poison could propagate, but undef must not.
+
 define <3 x i7> @all_ones_left_right_splat_poison_undef_elt(<3 x i7> %x) {
 ; CHECK-LABEL: @all_ones_left_right_splat_poison_undef_elt(
-; CHECK-NEXT:    [[LEFT:%.*]] = shl <3 x i7> <i7 poison, i7 -1, i7 undef>, [[X:%.*]]
-; CHECK-NEXT:    [[RIGHT:%.*]] = ashr <3 x i7> [[LEFT]], [[X]]
-; CHECK-NEXT:    ret <3 x i7> [[RIGHT]]
+; CHECK-NEXT:    ret <3 x i7> <i7 -1, i7 -1, i7 -1>
 ;
   %left = shl <3 x i7> <i7 poison, i7 -1, i7 undef>, %x
   %right = ashr <3 x i7> %left, %x
   ret <3 x i7> %right
 }
 
+; negative test - must have -1
+
 define i32 @almost_all_ones_left_right(i32 %x) {
 ; CHECK-LABEL: @almost_all_ones_left_right(
 ; CHECK-NEXT:    [[LEFT:%.*]] = shl i32 -2, [[X:%.*]]
@@ -330,6 +328,8 @@ define i32 @almost_all_ones_left_right(i32 %x) {
   ret i32 %right
 }
 
+; negative test - must have same shift amount
+
 define i32 @all_ones_left_right_not_same_shift(i32 %x, i32 %y) {
 ; CHECK-LABEL: @all_ones_left_right_not_same_shift(
 ; CHECK-NEXT:    [[LEFT:%.*]] = shl i32 -1, [[X:%.*]]