tcg-alpha: signed tlb comparison

Signed-off-by: Richard Henderson <rth@twiddle.net>
tcg-alpha: Add TCG_REG_TB

Signed-off-by: Richard Henderson <rth@twiddle.net>
tcg-alpha: Enable direct tb jumps

Signed-off-by: Richard Henderson <rth@twiddle.net>
tcg-alpha: Refactor emitting branch insns

Signed-off-by: Richard Henderson <rth@twiddle.net>
tcg-alpha: Remove stack slot for TB return

Two arithmetic insns are as fast or faster than a memory load
on all alpha implementations, and avoids the complication.
For USE_REG_TB, it will be a 2-insn offset from TCG_REB_TB;
otherwise it will be a 2-insn offset from TCG_REG_GP.

Signed-off-by: Richard Henderson <rth@twiddle.net>