msl: Only emit packed vectors when the width is 3

MSL vectors with other widths already match WGSL's rules for alignment
and size.

Change-Id: I237052372463ea8323eab47c3b4ca90c6d8afcc3
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/62600
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>

test/samples/compute_boids.wgsl.expected.msl

@@ -13,8 +13,8 @@ struct tint_symbol_3 {
   float4 value [[color(0)]];
 };
 struct Particle {
-  /* 0x0000 */ packed_float2 pos;
-  /* 0x0008 */ packed_float2 vel;
+  /* 0x0000 */ float2 pos;
+  /* 0x0008 */ float2 vel;
 };
 struct SimParams {
   /* 0x0000 */ float deltaT;