tcg: Fix tcg_reg_alloc_mov vs no-op truncation

Commit af3cbfbe8018ccc16fb3a0048e928f66f0d05e87 hoisted some "common"
loads of the temporary type, forgetting that the types could differ
during truncating moves.  This affects the correctness of the memory
offset on big-endian hosts.

Tested-by: Tom Musta <tommusta@gmail.com>
Signed-off-by: Richard Henderson <rth@twiddle.net>