Sema: invalid air when pointer casting from optional slice

[jacobly0]

fn foo(x: ?[]const u8) [*]const u8 {
    return @ptrCast(x);
}
comptime {
    _ = &foo;
}

$ zig build-obj repro.zig --verbose-air
# Begin Function AIR: repro.foo:
# Total AIR+Liveness bytes: 215B
# AIR Instructions:         7 (63B)
# AIR Extra Data:           10 (40B)
# Liveness tomb_bits:       8B
# Liveness Extra Data:      0 (0B)
# Liveness special table:   0 (0B)
  %0 = arg(?[]const u8, "x")
  %1!= dbg_stmt(2:12)
  %2 = optional_payload(?[]const u8, %0!)
  %3 = slice_ptr([*]const u8, %2!)
  %4 = bitcast([*]const u8, %3!)
  %5!= dbg_stmt(2:5)
  %6!= ret_safe(%4!)
# End Function AIR: repro.foo

[xdBronch]

related #21302