tcg/riscv64: Set vtype before whole-register vector loads

With vtype uninitialized, VILL might be set, leading to SIGILL.
Also, assert vtype initialized for whole-register stores and moves.

Fixes: f63e7089b4 ("tcg/riscv: Add basic support for vector")
Reported-by: Max Chou <max.chou@sifive.com>
Reviewed-by: Palmer Dabbelt <palmer@dabbelt.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
(cherry picked from commit 57d98debdf8a3a00e6022ad5c4dad1d8c4fa0f39)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>