[InstCombine] guard against splat-mul corner case

The test is already simplified, and I'm not sure how
to write a test to exercise the new clause. But it
protects the 2-bit pattern from miscompiling as noted
in D123453.

https://alive2.llvm.org/ce/z/QPyVfv
(If we managed to fall into the mul transform, it
would wrongly create a zero on this pattern.)

llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp

@@ -1165,11 +1165,11 @@ Instruction *InstCombinerImpl::visitLShr(BinaryOperator &I) {
 
     // Look for a "splat" mul pattern - it replicates bits across each half of
     // a value, so a right shift is just a mask of the low bits:
-    // lshr i32 (mul nuw X, Pow2+1), 16 --> and X, Pow2-1
+    // lshr i[2N] (mul nuw X, (2^N)+1), N --> and iN X, (2^N)-1
     // TODO: Generalize to allow more than just half-width shifts?
     const APInt *MulC;
     if (match(Op0, m_NUWMul(m_Value(X), m_APInt(MulC))) &&
-        ShAmtC * 2 == BitWidth && (*MulC - 1).isPowerOf2() &&
+        BitWidth > 2 && ShAmtC * 2 == BitWidth && (*MulC - 1).isPowerOf2() &&
         MulC->logBase2() == ShAmtC)
       return BinaryOperator::CreateAnd(X, ConstantInt::get(Ty, *MulC - 2));
 

llvm/test/Transforms/InstCombine/lshr.ll

@@ -399,6 +399,17 @@ define i32 @mul_splat_fold_no_nuw(i32 %x) {
   ret i32 %t
 }
 
+; Negative test (but simplifies before we reach the mul_splat transform)- need more than 2 bits
+
+define i2 @mul_splat_fold_too_narrow(i2 %x) {
+; CHECK-LABEL: @mul_splat_fold_too_narrow(
+; CHECK-NEXT:    ret i2 [[X:%.*]]
+;
+  %m = mul nuw i2 %x, 2
+  %t = lshr i2 %m, 1
+  ret i2 %t
+}
+
 define i32 @negative_and_odd(i32 %x) {
 ; CHECK-LABEL: @negative_and_odd(
 ; CHECK-NEXT:    [[TMP1:%.*]] = lshr i32 [[X:%.*]], 31