tcg/i386: Use SHLX/SHRX/SARX instructions

tcg/i386: Use SHLX/SHRX/SARX instructions

These three-operand shift instructions do not require the shift count
to be placed into ECX.  This reduces the number of mov insns required,
with the mere addition of a new register constraint.

Don't attempt to get rid of the matching constraint, as that's impossible
to manipulate with just a new constraint.  In addition, constant shifts
still need the matching constraint.

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Richard Henderson <rth@twiddle.net>