tcg: Optimize some forms of deposit.

tcg: Optimize some forms of deposit.

If the deposit replaces the entire word, optimize to a move.

If we're inserting to the top of the word, avoid the mask of arg2
as we'll be shifting out all of the garbage and shifting in zeros.

If the host is 32-bit, reduce a 64-bit deposit to a 32-bit deposit
when possible.

Signed-off-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>