expr: Allow scalar_int_mode target mode when converting a constant

expr: Allow scalar_int_mode target mode when converting a constant

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.

gcc/ChangeLog:

	* expr.c (convert_modes): Allow a constant integer to be converted to
	any scalar int mode.