[ir] Simplify lower swizzle view due to folding

Now that constant accesses are automatically folded in the builder (see:
https://dawn-review.googlesource.com/c/dawn/+/339936), this transform
can be slightly simplified to remove special-casing.

Bug: 551944362
Change-Id: Ib379d5a4ebebf490768fc29fc0f2271118d2dfdf
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/341875
Commit-Queue: Alan Baker <alanbaker@google.com>
Reviewed-by: Alan Baker <alanbaker@google.com>

src/tint/lang/core/ir/transform/lower_swizzle_view.cc

diff --git a/src/tint/lang/core/ir/transform/lower_swizzle_view.cc b/src/tint/lang/core/ir/transform/lower_swizzle_view.cc
index 1dc485c..bb2b13f 100644
--- a/src/tint/lang/core/ir/transform/lower_swizzle_view.cc
+++ b/src/tint/lang/core/ir/transform/lower_swizzle_view.cc
@@ -243,19 +243,12 @@
         return result;
     }
 
-    /// Maps an accessor index dynamically or statically to the target swizzle vector element index.
-    /// @param accessor_idx the accessor index (can be static constant or dynamic)
+    /// Maps an accessor index to the target swizzle vector element index.
+    /// @param accessor_idx the accessor index
     /// @param indices the collapsed swizzle indices
     /// @returns the mapped index value
     core::ir::Value* GetTargetIndex(core::ir::Value* accessor_idx,
                                     const tint::Vector<uint32_t, 4>& indices) {
-        // Index is constant.
-        if (auto* const_idx = accessor_idx->As<core::ir::Constant>()) {
-            uint32_t extra_idx = const_idx->Value()->ValueAs<uint32_t>();
-            return b.Constant(u32(indices[extra_idx]));
-        }
-
-        // Index is dynamic, and must be mapped into a composite array at runtime.
         tint::Vector<const core::constant::Value*, 4> const_indices;
         for (uint32_t idx : indices) {
             const_indices.Push(b.ConstantValue(u32(idx)));