revert "compiler: packed structs cache bit offsets"

This is mostly a revert of a7088fd9a3.
Measurement revealed the commit actually regressed performance.

src/Module.zig

@@ -6649,3 +6649,26 @@ pub fn structFieldAlignmentExtern(mod: *Module, field_ty: Type) Alignment {
 
     return ty_abi_align;
 }
+
+/// https://github.com/ziglang/zig/issues/17178 explored storing these bit offsets
+/// into the packed struct InternPool data rather than computing this on the
+/// fly, however it was found to perform worse when measured on real world
+/// projects.
+pub fn structPackedFieldBitOffset(
+    mod: *Module,
+    struct_type: InternPool.Key.StructType,
+    field_index: u32,
+) u16 {
+    const ip = &mod.intern_pool;
+    assert(struct_type.layout == .Packed);
+    assert(struct_type.haveLayout(ip));
+    var bit_sum: u64 = 0;
+    for (0..struct_type.field_types.len) |i| {
+        if (i == field_index) {
+            return @intCast(bit_sum);
+        }
+        const field_ty = struct_type.field_types.get(ip)[i].toType();
+        bit_sum += field_ty.bitSize(mod);
+    }
+    unreachable; // index out of bounds
+}