s390x: Support And1/Or1, improve handling of Int1 expressions

s390x: Support And1/Or1, improve handling of Int1 expressions

This provides an instruction selector for Int1-expressions that supports
And1 and Or1.  This implementation tries to keep values in registers as
much as possible, to avoid too many conversions from a Boolean value to a
condition code or vice versa.  To this end, the new function
s390_isel_int1_expr() is added, which handles bit-typed expressions that
are supposed to end up in a register.

Also change the representation of Int1 values in registers and always
sign-extend them to 64 bits.