(zero_extendhisi2, extendhisi2): Expand to suitable byte sequence when TARGET_SHORT_BYTES and loading a mem.

(zero_extendhisi2, extendhisi2): Expand to suitable byte sequence when
TARGET_SHORT_BYTES and loading a mem.
(movhi_bytes, extendhisi2_mem): New expands.
(movhi): Call movhi_bytes to load a short from memory if
TARGET_SHORT_BY_BYTES.
(all matchers loading a short from memory): Don't match if
TARGET_SHORT_BY_BYTES.
(matcher for movhi when TARGET_SHORT_BY_BYTES): New pattern to cover
remaining legal cases.

From-SVN: r8202