[llvm] [llvm][TypeSize] Consider TypeSize of '0' to be fixed/scalable-agnostic. (PR #72994)

[Paul Walker via llvm-commits]

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@@ -81,6 +81,23 @@ static_assert(INT64_C(2) * TSFixed32 == TypeSize::getFixed(64));
 static_assert(UINT64_C(2) * TSFixed32 == TypeSize::getFixed(64));
 static_assert(alignTo(TypeSize::getFixed(7), 8) == TypeSize::getFixed(8));
 
+static_assert(TypeSize() == TypeSize::getFixed(0));
+static_assert(TypeSize::getFixed(0) != TypeSize::getScalable(0));
+static_assert(TypeSize::getFixed(0).isZero());
+static_assert(TypeSize::getScalable(0).isZero());
+static_assert(TypeSize::getFixed(0) ==
+              (TypeSize::getFixed(4) - TypeSize::getFixed(4)));
+static_assert(TypeSize::getScalable(0) ==
+              (TypeSize::getScalable(4) - TypeSize::getScalable(4)));
+static_assert(TypeSize::getFixed(0) + TypeSize::getScalable(8) ==
+              TypeSize::getScalable(8));
+static_assert(TypeSize::getScalable(8) + TypeSize::getFixed(0) ==
+              TypeSize::getScalable(8));
+static_assert(TypeSize::getFixed(8) + TypeSize::getScalable(0) ==
+              TypeSize::getFixed(8));
+static_assert(TypeSize::getScalable(0) + TypeSize::getFixed(8) ==
+              TypeSize::getFixed(8));
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paulwalker-arm wrote:

It's worth repeating this test for `-` to exercise the change to `operator-=`.

https://github.com/llvm/llvm-project/pull/72994