accel/tcg: Reorg translator_ld*

accel/tcg: Reorg translator_ld*

Reorg translator_access into translator_ld, with a more
memcpy-ish interface.  If both pages are in ram, do not
go through the caller's slow path.

Assert that the access is within the two pages that we are
prepared to protect, per TranslationBlock.  Allow access
prior to pc_first, so long as it is within the first page.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>