[x86_64] shift operations may return wrong result if 2nd operand is larger than int8

Shifting instructions in Mu require two operands of the same size, e.g. Shl <int64> a b, in which a and b are both int64.

However shl, shr, sar in x86_64 either takes a second operand in the CL register (8 bits), or as a 8bits immediate. Current the instruction selector simply moves lower 8bits of b into CL, which may result in incorrect result.