is_int_mode does not allow MODE_PARTIAL_INT modes, so convert_modes was
not allowing a constant value to be converted to a MODE_PARTIAL_INT for
use as operand 2 in patterns such as ashlpsi3. The constant had
to be copied into a register before it could be used, but now can be
used directly as an operand without any copying.

Successfully regtested on trunk for x86_64-pc-linux-gnu and msp430-elf.
Ok to apply?