[ADT] Fix a bug in PackedVector::setValue for signed types (llvm#159239)

Without this patch, we forget to update the sign bit.  When we assign:

  Vec[0] = -1;

the sign bit is correctly set to 1.  Overwriting the same element:

  Vec[0] = 1;

does not update the sign bit, leaving the 4-bit as 0b1001, which reads
-2 according to PackedVector's encoding.  (It does not use two's
complement.)

This patch fixes the bug by clearing the sign bit when we are
assigning a non-negative value.

Author: kazutakahirata

llvm/include/llvm/ADT/PackedVector.h

@@ -58,6 +58,8 @@ class PackedVectorBase<T, BitNum, BitVectorTy, true> {
     if (val < 0) {
       val = ~val;
       Bits.set((Idx * BitNum) + BitNum - 1);
+    } else {
+      Bits.reset((Idx * BitNum) + BitNum - 1);
     }
     assert((val >> (BitNum-1)) == 0 && "value is too big");
     for (unsigned i = 0; i != BitNum-1; ++i)

llvm/unittests/ADT/PackedVectorTest.cpp

@@ -71,6 +71,14 @@ TEST(PackedVectorTest, RawBitsSize) {
   EXPECT_EQ(12u, Vec.raw_bits().size());
 }
 
+TEST(PackedVectorTest, SignedValueOverwrite) {
+  PackedVector<signed, 4> Vec(1);
+  Vec[0] = -1;
+  EXPECT_EQ(-1, Vec[0]);
+  Vec[0] = 1;
+  EXPECT_EQ(1, Vec[0]);
+}
+
 #ifdef EXPECT_DEBUG_DEATH
 
 TEST(PackedVectorTest, UnsignedValues) {