target/i386: add X86_SPECIALs for MOVSX and MOVZX

target/i386: add X86_SPECIALs for MOVSX and MOVZX

Usually the registers are just moved into s->T0 without much care for
their operand size.  However, in some cases we can get more efficient
code if the operand fetching logic syncs with the emission function
on what is nicer.

All the current uses are mostly demonstrative and only reduce the code
in the emission functions, because the instructions do not support
memory operands.  However the logic is generic and applies to several
more instructions such as MOVSXD (aka movslq), one-byte shift
instructions, multiplications, XLAT, and indirect calls/jumps.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>