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 <[email protected]>
Reviewed-by: Aurelien Jarno <[email protected]>
Signed-off-by: Richard Henderson <[email protected]>