target/arm: Simplify GEN_SHIFT in translate.c

target/arm: Simplify GEN_SHIFT in translate.c

Instead of computing

    tmp1 = shift & 0xff;
    dest = (tmp1 > 0x1f ? 0 : value) << (tmp1 & 0x1f)

use

    tmpd = value << (shift & 0x1f);
    dest = shift & 0xe0 ? 0 : tmpd;

which has a flatter dependency tree.
Use tcg_constant_i32 while we're at it.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>