IR: Simplify types. F32 -> U32, F64 -> U64, F128 -> U128

ARM's Architecture Specification Language doesn't distinguish between floats and integers
as much as we do. This makes some things difficult to implement. Since our register
allocator is now capable of allocating values to XMMs and GPRs as necessary, the
Transfer IR instructions are no longer necessary as they used to be and they can be
removed.

src/backend_x64/reg_alloc.cpp

@@ -64,11 +64,7 @@ static size_t GetBitWidth(IR::Type type) {
             return 32;
         case IR::Type::U64:
             return 64;
-        case IR::Type::F32:
-            return 32;
-        case IR::Type::F64:
-            return 64;
-        case IR::Type::F128:
+        case IR::Type::U128:
             return 128;
         case IR::Type::NZCVFlags:
             return 32; // TODO: Update to 16 when flags optimization is done