(zero_extendhisi2): Rewrite to work if op0 == op1.  Use
gen_lowpart in preparation code.
(extendqisi2, extendhisi2): Likewise.
(extendqihi2): Expand.
(restorehi): Rewrite for correctness, efficiency, and
clarity. Avoid generating insns involving truncate.  Generate
needed pseudos in preparation statements.
(storehi): Likewise.
(storeinthi): Likewise.
(movhi): Call gen_storehi, gen_restorehi, gen_storeinthi according
their new definitions.  Use force_reg to put address in register.

From-SVN: r2255