tcg/tci: Remove TCG_CONST

Restrict all operands to registers.  All constants will be forced
into registers by the middle-end.  Removing the difference in how
immediate integers were encoded will allow more code to be shared
between 32-bit and 64-bit operations.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>