target-mips: simplify LWL/LDL mask generation

The LWL/LDL instructions mask the GPR with a mask depending on the
address alignement. It is currently computed by doing:

    mask = 0x7fffffffffffffffull >> (t1 ^ 63)

It's simpler to generate it by doing:

    mask = ~(-1 << t1)

It uses one TCG instruction less, and it avoids a 32/64-bit constant
loading which can take a few instructions on RISC hosts.

Cc: Leon Alrae <leon.alrae@imgtec.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Reviewed-by: Leon Alrae <leon.alrae@imgtec.com>
Signed-off-by: Leon Alrae <leon.alrae@imgtec.com>